Given the current status of Flow, and because there are issues with enabling/disabling Flow when a user is blocked/unblocked, I propose that if a user doesn't currently have Flow enabled on their talk page, we should prevent them from enabling it.
User cases to consider for current Flow users (from T335772):
- A user who creates a new account would not have SD listed as an option in Beta Features.
- An existing user who never turned on SD or who turned SD off would not have SD listed as an option in Beta Features.
- An existing user with SD being active at their talk page would have an option in Beta Features to turn the feature off definitively.