The reason support stops for older versions of ROMs is because it is 1 step behind the newer one. The newest one have more issues fixed, more features, and more compatibilities than the older.
Plus, if you keep old ones around, it will confuse a lot of new users thinking "hey, I like Liberty 1.0, and I like this or that in Liberty 1.5, let's see if I can mix it together." Then the end result is nothing but problems, boot loops, and much more. It's just better it finalize certain ROMs and keep everything together that is compatible for that ROM and kept separate from the rest or drop it altogether. And if you're continuing off the same ROM but adding to it, fixing issues, and tweaking things then it's better to drop support for the older versions and eliminate altogether, or parts of it if the other parts are still compatible with the newer stuff.