Tweetbot, stopped working without warning and without any apparent explanation from the company, with developers of such applications being unable to contact anyone to source an explanation for their errors, which often include a reference to authentication being unavailable or being unauthorised. Later, Twitter announced it was "enforcing long-standing API rules" and retroactively changed its Developer Terms of Service afterwards to justify its unannounced banning of third-party Twitter clients. In February, Twitter announced that it would be removing the free tier of its API within a week and replacing it with a "basic paid tier", effectively shutting down the many volunteer bots and applications that use the platform, such as StreamerBans and Video Downloader bots. Two days after this announcement, The Information reported plans by Twitter that any business account wanting to keep their gold checkmark as verification of their business will have to pay a minimum of $1000 per month, plus $50 for every affiliate account they include as part of their "business", such as accounts for sub-divisions of the company or even notable employees. On February 15, 2023, Twitter announced that SMS two-factor authentication for Twitter accounts will only be available to Twitter Blue subscribers starting March 20, 2023. Appearance and features Tweets Tweets are publicly visible by default, but senders can restrict message delivery to only their followers. Users can mute users they do not wish to interact with, block accounts from viewing their tweets, and remove accounts from their followers list. Users can tweet via the Twitter website, compatible external applications (such as for smartphones), or by Short Message Service (SMS) available in certain countries. Users may subscribe to other users' tweets—this is known as "following" and subscribers are known as "followers" or "tweeps", a portmanteau of Twitter and peeps. Individual tweets can be forwarded by other users to their own feed, a process known as a "retweet". In 2015, Twitter launched "quote tweet" (originally called "retweet with comment"), a feature that allows users to add a comment to their retweet, nesting one tweet in the other. Users can also "like" (formerly "favorite") individual tweets. The counters for "likes", "retweets", and replies appear next to the respective buttons in timelines such as on profile pages and search results. Counters for likes and retweets exist on a tweet's standalone page too. Since September 2020, quote tweets, formerly known as "retweet with comment", have their own counter on