LineageOS

LineageOS is an operating system for smartphones, tablet computers, and set-top boxes, based on Android with mostly free and open-source software. It is the successor to the custom ROM CyanogenMod, from which it was forked in December 2016 when Cyanogen Inc. announced it was discontinuing development and shut down the infrastructure behind the project. Since Cyanogen Inc. retained the rights to the Cyanogen name, the project rebranded its fork as LineageOS.
LineageOS was officially launched on 24 December 2016, with the source code available on both GitHub and GitLab. As with all versions of Android, operating system releases are specific to a single device model. Since its launch, LineageOS development builds are available for 109 phone models with over 2.8 million active installs, having doubled its user base in the months February–March 2017.
Background.
CyanogenMod (often abbreviated "CM") was a popular open-source operating system for smartphones and tablet computers, based on the Android mobile platform. Although only a subset of total CyanogenMod users elected to report their use of the firmware, as of 23 March 2015, some reports indicated over 50 million people running CyanogenMod on their phones. It was also frequently used as a starting point by developers of other ROMs.
In 2013, the founder, Stefanie Kondik, obtained venture funding under the name Cyanogen Inc. to allow commercialization of the project. In her view, the company did not capitalize on the project's success and in 2016 she either left or was forced out as part of a corporate restructure which involved a change of CEO, closure of offices and projects, and cessation of services. The code itself, being both open source and popular, was quickly forked under the new name LineageOS and efforts began to resume development as a community project.
CyanogenMod offered a number of features and options not available in the official firmware distributed by most mobile device vendors. Features supported by CyanogenMod included native theme support, FLAC audio codec support, a large Access Point Name list, "Privacy Guard" (per-application permission management application), support for tethering over common interfaces, CPU overclocking and other performance enhancements, root access, soft buttons and other "tablet tweaks," toggles in the notification pull-down (such as Wi-Fi, Bluetooth and satellite navigation), and other interface enhancements. Many of the features from CyanogenMod were later integrated into the official Android code base. CyanogenMod's developers said that it did not contain spyware or bloatware. CyanogenMod was also said to perform better and be more reliable than official firmware releases.
Development.
Like CyanogenMod, the LineageOS project is developed by many device-specific maintainers and uses Gerrit for its code review process. It also retained the old versioning format (for example, Android 7.1 is LineageOS 14.1). 
Prior to the official launch of LineageOS, many developers from XDA had already developed unofficial versions of LineageOS from the source code.
All the released builds are signed with LineageOS' private keys.
Builds were released on a weekly basis until 12 November 2018, when the release cycle for devices has changed: the latest LineageOS branch is built daily, with devices receiving a "nightly" OTA update, while devices on the older branch were moved to a weekly release cycle.
Community.
LineageOS allows the community to get involved with development in various ways. Gerrit is used for the code review process for both the operating system and the infrastructure.
The wiki, containing information regarding installation, support, and development of LineageOS, is also open to contributions through Gerrit. Other Lineage platforms include Crowdin for managing translations, Gitlab Issues for bug tracking, and a stats page, which displays the number of active installations from users who opt in to report this statistic. There is also an official subreddit, r/lineageos, and two IRC channels hosted on Freenode ("#lineageos" and "#lineageos-dev").
The XDA Developers forums have been used by members of the Lineage community since the software's inception. Many devices are left unsupported by official releases so community members develop their own unofficial ROMs allowing older phones to use Lineage. These unofficial releases are often bundled with software intended to aid the user's experience that would otherwise be unseen in an official release. They also come with known bugs and security issues that may not be seen in official releases.
During August 2017 the LineageOS team held a Summer Survey in which they asked users for feedback to improve the development of the operating system. The results were published in October and, according to the team, they used the gathered data to improve the upcoming LineageOS 15 release. A second Summer Survey was conducted in August 2018.
As a response to one of the main suggestions received during their first public survey, LineageOS launched a section on their blog titled "LineageOS Engineering Blog" where Lineage maintainers and developers can contribute articles discussing advanced technical information pertaining to Android development.
LineageOS is also known for posting a "regularly irregular review" on its blog in which the active development of the work is discussed.
Preinstalled apps.
LineageOS includes many essential and useful apps, but like its predecessor, CyanogenMod, it is free from unnecessary software often pre-installed by a phone's manufacturer or carrier that is considered to be bloatware.
Current
Former 
Although they are not included in LineageOS as such due to legal issues, users can flash the normal Google apps, including the Google Play Store and Play Apps, with a Zip package, usually referred to as "gapps", while installing LineageOS.
Unique features.
LineageOS offers several features that Android Open Source Project (AOSP) does not include. Some of these features are:
Trust interface.
As LineageOS evolved through development, the Trust interface was introduced for all the LineageOS 15.1 builds released since 12 June 2018. The interface can be found on supported devices under Security and Privacy tab under the Settings option, and enables the user to "get an overview of the status of core security features and explanations on how to act to make sure the device is secure and the data is private".
Additionally, while carrying out any action on the device, the trust icon is displayed, notifying the user that the action is safe.
Supported devices.
The number of devices supported by LineageOS has increased over time, with 157 for 17.1 and 18.1 . Official builds on currently supported development branches are labeled as "nightly". For the first two months of the project, parallel experimental builds were also produced, allowing in-place upgrades from previous CyanogenMod installations and easing migration to LineageOS.
Criticism and reception.
2018 April Fools' prank.
LineageOS was criticized for a deceptive April Fool's prank included with some April 2018 builds.
During the first week of April 2018 LineageOS released new builds with the "LOSGenuine" prank that informed unaware users of the software possibly being counterfeit via a persistent notification (which could not be disabled unless the user ran the following command in a root shell):
setprop persist.lineage.nofool true 
When the notification was tapped, the software claimed that the device was "uncertified" and needed to mine "LOSCoins", which were a virtual currency and could not actually be spent. Affected builds also had a preinstalled "Wallet" app that showed the current balance of LOSCoins.
Many users mistook the prank for actual malware, and others reportedly found it to be in "poor taste". It was especially criticized for being too "late" for an April Fool's joke, since many users didn't receive the update until days later, making the jest less obvious. On 10 April 2018, LineageOS team director "ciwrl" issued an official apology for the deceptive prank.
Forks.
Replicant is a completely free software variant of LineageOS, with all kernel blobs and non-free drivers removed.
As a response to the refusal for several reasons of support for signature spoofing in official builds, a LineageOS fork with microG services included, known as "LineageOS for microG", was created. The project ships custom builds of LineageOS with the required patch and native F-Droid support, bundled with the MicroG project's free re-implementation of proprietary Gapps. In other respects it follows upstream, shipping OTA updates every seven days.
/e/ is a fork of LineageOS created by Gaël Duval that is intended to be "free from Google". It replaces Google Play Services with MicroG, a free and open-source implementation of Google APIs.