Structured Discussions are not developed, and they are minimally maintained.
At some wikis, it is still possible for any user to turn SD on at their talk page, using an option in Beta features.

The idea is to prevent new Flow boards from being created by users, as we plan to sunset this extension.

User cases:

• 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.