I suspect this is because we didn’t drop the index concurrently.
A normal
DROP INDEXacquires anACCESS EXCLUSIVElock on the table, blocking other accesses until the index drop can be completed.
This should be fixed in PERF: Update migration to drop index concurrently. by tgxworld · Pull Request #28955 · discourse/discourse · GitHub.
@schneeland Thank you for taking the time to report the issue 