SERVER-106222 added an optional isViewlessTimeseries flag flag to oplog entries that gets set every time a write happens on a collection with timeseries options and a namespace not in the system.buckets format.

This ticket is a placeholder to flag some potentially problematic scenarios that may need to be handled:

• How to treat such oplog entries on downgrade since they're not backwards compatible? Should the oplog be proactively trimmed when setting the FCV to the previous version?
• The oplog field addition is not feature flagged, so wrongly formatted timeseries collections (not prefixed with system.buckets) that may have timeseries options are going to be identified as viewless timeseries, potentially leading to crashes in multiversion scenarios (SERVER-109087 pointed out that rename could have created such kind of collections, it is at least one known way to get into such state).