Notes:

• the bug is not happening in group 1 wiki, e.g. cawiki
• the bug was found when checking T248309: StructuredDiscussions (Flow): Prevent users from enabling Flow on their user talk page (beta feature) for group 0 in wmf.10
• the bug was not present in beta (cawiki beta)
• the existing Flow boards' functionality is not affected.

Steps to reproduce

1. As a user with active Flow board, go to Preferences - Beta on testwiki wmf.10 (or any other wiki with enabled Flow in group zero)
2. The option for beta feature Structured Discussions on user talk is not present, so a user cannot turn it off.