Sad to see yet another case of transaction ID wraparound in causing very long outage status.coveralls.io
It's important to monitor the horizons both for XID and multixact ID (check if you do the latter), have reliable alerts, and fight long-running transactions.
Conversation
While this is bad, and should not be minimized, one piece of practical advice occurs to me: They should use a manual VACUUM with INDEX CLEANUP=off (available in 12+).