This bug has been split off #9930.

With the arrival of coroutines defer.inlineCallbacks and defer.deferredGenerator no longer serve a purpose, as coroutines are faster and more convenient to use.

#12239 added a convenient migration path from defer.inlineCallbacks to coroutines.

I think #12239 should be released first without deprecating defer.inlineCallbacks. Then wait some time, possibly years, and only then deprecate defer.inlineCallbacks. This way people will be able to support older versions of Twisted, e.g. as shipped in Linux distributions, without spending time to work around warnings. They would need to do so because they still support old Twisted, and thus can't use features of new Twisted anyway.