Allowing time-series collections to be sharded on the time field adds complexity and special handling for time-series in code.

Because a time-series bucket contains a range of time and may overlap it is impossible without certain restrictions (such as fixed bucketing) to determine which shard owns a particular time.

It makes building features such as unique indexes for time-series collections more difficult and inefficient.

Users with large time-series workloads typically have (very) high cardinality and sharding on meta is sufficient.

By deprecating support for sharding time-series collections on the time field, we can remove it in the future to allow for simplifications of the code.