Because v3 is fully compatible with v1, there's no hybrid between the two. Both v1 and v3 clients already connect and exchange pieces directly, and use the same hash-ID. Think of it as v3 is already v1+v3.
The private v3.1 would not be private if it was be combined in a hybrid torrent with v1 or v2. This would defeat it's purpose.
More info:
https://tixati.com/specs/bittorrent/v3
https://tixati.com/specs/bittorrent/v3.1