We are introducing unique index data format 13 and 14 on 6.0. As a result, we would also allow the new data formats to exist on 5.0 for downgrade compatibility. However, 5.0 will be the downgrade floor, and we will still use only data format 11 and 12 when creating and modifying indexes on 5.0.

Also, the master branch code will need the support on 5.0 to pass the multiversion tests.