Would have chosen issue-type "task" rather than "bug" if the issue tracker would have given me a choice...

Besides introducing many nifty new features, MC 1.9 is fixing many bugs discovered in earlier versions. While this will help with new worlds created in 1.9, there are many well-populated servers running worlds in 1.8 and even 1.7 that can't or won't convert to a new major release. Those continuing server worlds are where many MC veterans introduce Minecraft to new players, so it behooves Mojang to still patch bugs if possible.

Therefore, Mojang should backport as many of 1.9's bug fixes as practical to the version(s) where they arose. Concurrent with polishing 1.9 for release, Mojang should issue "final" patches (e.g. mc-1.8.9 and mc-1.7.11) to support legacy servers.