Where are you seeing this? The 'limit' parameter works correctly for me on master, and I don't see what in the code could make it not work.

I can replicate it at http://en.wikipedia.beta.wmflabs.org/wiki/Special:RecentChanges currently, as well on my local development environment where I pinpointed it with git bisect.

I can confirm on both translatewiki and my own test wiki. (I created 500 pages using maintenance/edit.php and only 50 show up.)

The problem is that SpecialRecentchanges::getFeedQuery() is called when generating the feed link, causing the limit parameter to be validated against $wgFeedLimit, using the following:

$this->getOptions()->validateIntBounds( 'limit', 0, $wgFeedLimit );

A workaround is to clone the FormOptions object first.

Bah, and actually I just recently noticed that this is an issue when writing https://gerrit.wikimedia.org/r/#/c/110864/ .

Change 111201 had a related patch set uploaded by Bartosz Dziewoński:
Don't override the 'limit' parameter when building feed query

https://gerrit.wikimedia.org/r/111201

Change 111201 merged by jenkins-bot:
Don't override the 'limit' parameter when building feed query

https://gerrit.wikimedia.org/r/111201

No need to backport anything, the troublesome commit was not included in any WMF branch yet.