5.0.7 and 5.3.2 will be the downgrade floor for 6.0. Earlier versions of 5.0.x and 5.3.x will not recognize the new unique index versions and would fail to start.

Be sure to update the 6.0 Compatibility Notes with downgrade information.

Description of Linked Ticket

We can do this by introducing a new internal data format version and use that when creating new unique indexes.

In the future, this new data format will guarantee that no legacy format secondary unique index entries are still around.