We just had a discussion in #mediawiki about this commit:
http://www.mediawiki.org/wiki/Special:Code/MediaWiki/76064
It removed wfLoadExtensionMessages() from dozens of extensions in trunk. With trunk MediaWiki, this causes no problems, since the function does nothing. But in older MediaWiki versions, it will cause the extension to break. Yaron reported that he quickly got a complaint from someone using one of his trunk extensions together with MediaWiki 1.15.
This kind of thing has come up before, e.g., the removal of IE5 support from Monobook. My take on this is that the cost-benefit analysis is very simple:
* Cost to users of removing wfLoadExtensionMessages() from trunk extensions: If the extension happened to work on some old MW version (some do), the extension breaks. * Benefit to users of removing wfLoadExtensionMessages() from trunk extensions: Nothing. Extension works the same as before, no better or worse.
I think it's very clear that in cases like this, we should not remove back-compat. If there's some benefit, then maybe -- for instance, if the code is hard to maintain, that could outweigh back-compat benefits. But this code isn't hard to maintain at all, it's one line in each extension. If there's no cost, then sure -- for instance, if the extension already only works on trunk for some other reason (maybe a new feature was added that uses a trunk function), then no harm is caused by removing the back-compat lines.
But going out of your way to remove back-compat when that will hurt some users and not ease development *quantifiably* does not make any sense, and IMO no one should be doing that. ("Quantifiably" means "I'm not going to bother adding new feature X if I have to maintain back-compat while doing it", not "I think the code is uglier".) We can disagree about how much work we need to go into to maintain back-compat, but it's not *negative* -- putting work into removing back-compat without clear gain does not make sense.
I've created a page on mediawiki.org to set out some guidelines we can consider: http://www.mediawiki.org/wiki/Backward_compatibility. I think we should get something written down that we can agree on, since this issue has come up more than once before and the same people tend to repeat the same disagreements every time.