Thank you for the follow-up. In mean-time I can confirm the commit 77dff5d937b1 might be the source of the original reported issue.
Unfortunately pinning version down to 16.12 only avoids the MultiXactOffsetSLRU self-deadlock, but the standby then fails recovery after 12+ hours.
FATAL: could not access status of transaction 24958976
DETAIL: Could not read from file "pg_multixact/offsets/017C" at offset 221184: read too few bytes.
CONTEXT: WAL redo at 14770/873268E8 for MultiXact/CREATE_ID: 24958975 offset 61500431 nmembers 2: 3058927188 (fornokeyupd) 3058927189 (keysh)
We are going to try to pin 16.13 and try that before we can safely upgrade of the primary/are confident we have working PITR recovery available should we need it.
Radim
PS: Once I have some time I will try to setup a docker based harness to be able to replicate original problem for later testing of the fix.