In SERVER-114496 we forbid all commands targeting legacy system.buckets collections in FCV 9.0, but only for external (user-facing) commands. SERVER-101597 extends this to forbid indirect commands.

We should also add a guardrail that forbids all commands (including internal) from creating the wrong type of timeseries collection based on the state of the featureFlagCreateViewlessTimeseriesCollections feature flag. This guardrail should be implemented at the low level in the create collection path, so that no codepath can bypass it. Specifically:

• When featureFlagCreateViewlessTimeseriesCollections is enabled: forbid creation of viewful (legacy) timeseries collections.
• When featureFlagCreateViewlessTimeseriesCollections is disabled: forbid creation of viewless timeseries collections.

Currently, _createTimeseriesCollection() already has a tassert that checks for legacy system.buckets namespace when the feature flag is enabled. This should be strengthened to a uassert and extended to also cover the reverse case (viewless creation when the flag is disabled).