No longer maintained / barely changing, like MS-DOS, Mac OS 9, Amiga OS (which does occasionally get an update but usually not much in the way of core changes)
It's relevant because Windows is a moving target and the dead operating systems aren't so there's not a lot of support work needed for them nor will there be as much of a user base.
How many of them are supported in mainline Curl, aren't POSIX and aren't dead operating systems?