Hi,
After upgrading my site's MediaWiki install to 1.21rc1, we noticed the MobileFrontend (at master) was throwing a fatal error. I mentioned it in #wikimedia-mobile and Ori found that it depends on a core change that was only merged recently (
https://gerrit.wikimedia.org/r/#/c/49071/).
Since MobileFrontend is considered to be stable, it would be nice if there was a REL1_21 branch so that we could get the features that have been introduced since REL1_20 was branched, and that don't require 1.22.