tl;dr: All wikis are staying at 1.28.0-wmf.18 for now
Last week all wikis were rolled back to MediaWiki version 1.28.0-wmf.18
due to several problems that were spotted on Friday (2016-09-16)[0][1].
The problems with wmf.19 seemed resolved by Monday (2016-09-19). The
plan was to roll wmf.19 out to all wikis yesterday afternoon and continue
with the wmf.20 branch-cut today as scheduled.
Late yesterday a large performance regression was discovered in
wmf.18[2].
We've paused the rollout of wmf.19 and the branching of wmf.20 to allow
time to investigate the nature of this performance regression.
After there is a better understanding of the performance regression, we
will reevaluate our plans for branching and rollout of wmf.19 and wmf.20.
-- Tyler
[0].
https://phabricator.wikimedia.org/T111441
[1].
https://phabricator.wikimedia.org/T145819
[2].
https://phabricator.wikimedia.org/T146099