Silencing "Your disk is almost full" notification
The solution to disabling the "almost full" and "full" notification is to disable the daemon responsible for it:

launchctl unload -w /System/Library/LaunchAgents/com.apple.diskspaced.plist


or

launchctl stop com.apple.diskspaced


Alternatively, if you only want to prevent the "almost full" from appearing so often then you can lower the GB threshold via:

minFreeSpace (int) - minimal free size in GB. Default: 20


The default 20GB is too high for small SSDs and a possible bug causes the alert to be shown every day rather than just once, so as a workaround you can lower the free space before the alert appears, e.g. to 10GB:

defaults write com.apple.diskspaced minFreeSpace 10


The daemon only reads its prefs on startup so you need to restart it if you have system integrity turned off:

launchctl unload -w /System/Library/LaunchAgents/com.apple.diskspaced.plist
launchctl load -w /System/Library/LaunchAgents/com.apple.diskspaced.plist


Otherwise kill it:

killall diskspaced


In case you are interested in the other preferences for these disk alerts you can view some of them using the help param:

/System/Library/PrivateFrameworks/StorageManagement.framework/Versions/A/Resources/diskspaced help
---
  Domain: com.apple.diskspaced
  Supported keys:
  debugLog (BOOL) - log additional debug information. Default: NO
  checkAllVolumes (BOOL) - check all volumes. Default: NO
  minDiskSize (int) - minimal disk size in GB. Default: 128
  minFreeSpace (int) - minimal free size in GB. Default: 20
  minPurgeableSpace (int) - minimal purgeabe space size in GB. Default: 20
---
  Commands: removeAllNotifications - Removes all scheduled and delivered user notificiations.


And here are a couple of hidden ones:

warningInterval (integer default 0)
lastWarningDate (string e.g. 2017-05-05 16:48:29 +0000)


I didn't look too closely at but it is possible setting the last warning date to a date in the future would also prevent the alert displaying.