Thunderbird and Movemail
According to the Thunderbird 87 change log, Movemail support was removed.
Another classic WTF decision.
Cripple software rather than build useful software. Make life easier for developers rather than users.
Sure, Linux users will find a way to work around the breakage. While a majority of users do not use 90% of the available features in any software, removing well established features does nothing but damage confidence from long time and experienced users. Cynicism increases. Trust dwindles.
Modern software development continues to erode downhill.