charlie.swanson@mongodb.com pointed out in SERVER-106933 that we have a gap in coverage for the scenario where an election takes place before a repl-set upgrade is complete. That could get us into a mixed version scenario where the primary has completely upgraded, but one of the secondaries is still on the previous version. Without FCV-gating on the query feature (as is in extensions), it may allow the primary to write the stage to the view catalog; that could cause an fassert on the secondary. This scenario should be investigated and tested.

We should also look into scenarios where we use read preference secondary throughout the upgrade/restart sequence.

Overall, we should think hard to make sure we have well-defined coverage of ReplSet upgrade/restart, not just where we're connected to the primary node the whole time.