I just noticed that ContentHandler has understandably deprecated several hooks.
However, not all the hooks have been updated as deprecated on the mw.o. Some of their replacements aren't even documented on mw.o.
Worse, there isn't any schedule for how long the legacy hooks are going to be supported or any HOWTOs for developers interested in making their extensions work in MW 1.21+..
This is especially frustrating for developers who may have used ArticleViewCustom (new in 1.19) only to have it be deprecated a year later in 1.21 for ArticleContentViewCustom.
(If anyone did use this hook in 1.19, they didn't document that use on mw.o, so we don't know about it and can't warn them about the issues that may come from this.)
Mark.