tl;dr: If your team doesn't do any development on GitHub then this email likely doesn't affect you.
As you may or may not know there is now a read-only replica of Gerrit available at https://gerrit-replica.wikimedia.org/ (hooray); however, over the weekend we noticed some missing tags from that mirror (boo).
To fix the missing tags for the replica I forced replication to run for all repos in Gerrit today as part of a configuration restart. After a replication sync I was able to ensure that all repos on the new replica were now up-to-date; however, it also closed all the pull requests that were made via pushing branches to wikimedia-org GitHub repos (which is the work flow of several apps teams and possibly others).
Apologies for the inconvenience and thanks to Dmitry Brant and Joe Walsh for pinging me about the problem.
I've since removed GitHub as a "mirror" -- meaning Gerrit will not delete branches there. Paladox has filed a task upstream to allow us to specify a full replication for a particular remote (i.e., gerrit-replica but not GitHub) instead of all remotes[0], and for added suspenders for our belt I've made a patch set that should exclude these projects from replicating to from Gerrit to GitHub in the future[1].
I think all of the fallout of this change is taken care of (judging from my GitHub search): https://github.com/search?p=1&q=org%3Awikimedia+is%3Aunmerged+type%3Apr+updated%3A%3E%3D2019-08-05+is%3Aclosed&type=Issues
But if your project was affected, please either reach out to me or add your project to the GitHub exclusion list in Puppet like in my patchset[1] and add me as a reviewer.
Thanks and sorry -- Tyler
[0]. https://bugs.chromium.org/p/gerrit/issues/detail?id=11280 [1]. https://gerrit.wikimedia.org/r/c/operations/puppet/+/528276