The index data format version currently depends on three things:

1. uniqueness
2. index version (v1 or v2)
3. key/value format

However, uniqueness itself does not actually directly affect the format of the data in the index as that can be entirely determined by the index version and the key/value format (i.e. whether the record id is in the keys or in the values, or a mix of both). Making it independent of uniqueness would allow unique index conversions to no longer need to modify the version number, which would alleviate the issues linked to SERVER-68216.