Comment 23 for bug 1759628

"Mature" means it hasn't changed in a while and we know what bugs it contains. "Mature" does not mean zero bugs, but at least by not changing it at the last minute we don't risk introducing bigger bugs. And this bug is not big at all.

As an example, just today the "gjs" package has blown up with lots of new crashes that didn't exist last week. This was caused by an unnecessary upgrade of a previously mature package, and it looks like it may risk the whole 18.04 release. That is why you shouldn't change things late in the release cycle.