Note: Before deploying wmf.31 to group 2 (e.g. frwiki wmf.30; check List of wikis with Structured discussion feature- enabling/disabling Structured discussion worked fine.
On any wiki with wmf.31 (or in enwiki betalabs) - try to enable/disable 'Structured Discussions on user talk' beta feature - it will have no effect on a User talk page.
- if a User talk page did not exist before- it won't be created
- (enabling the feature) if a user page was in text format, it won't convert to Structured Discussions format
- (disabling the feature) if a user page was in Structured Discussion format, it won't be converted to the text format.
"Restore all default settings (in all sections)" won't have any effect on an existing Structured discussion User talk page - the db won't have a record of up_property: beta-feature-flow-user-talk-page up_value: 1, but the SD page is still present.