As currently implemented, we consider a collection viewless timeseries if it has timeseries options and it's on a not a system.buckets namespace.

However historically we have allowed users to rename system.buckets collections to non-system.buckets collection, this was prevented in SERVER-79630.

If a user still has an historical inconsistency, under the current logic it's considered viewless timeseries and may trigger incompatible behavior even in fully downgraded FCV (e.g. setting the isViewlessTimeseries oplog field).

We should properly distinguish viewless timeseries from buckets inconsistencies.