I’d like to see enabling the U2F (or any other 2FA method) treated like a change in the master password and request all the bitwarden clients already authenticated to re-authenticate. I think this behaviour is the best practice from the security point of view.
It is my understanding that if the authentication method changes from 1 factor to 2 factors all clients must be re-authenticated so that all have the same level of authenticity. Currently, it is possible to have clients that coexist with 1 factor and others with 2 factors. The request of all clients to re-authenticate after enabling U2F/2FA is common on other services such as Google accounts and Lastpass.