From bug T62748.
> However, the current code will only enable the new betafeatures when the user visits Special:Preferences, or whenever the 'GetPreferences' hook is run (API's action=options I think).
Needs to handle metrics properly (see comments).
1. Keep track of date "Automatically enable..." was set to "yes" in a hidden preference/somewhere.
2. Store the date each beta feature was launched (may very by wiki) in config.
3. Use UserGetDefaultOptions. Set the default to "enabled" for any Beta Feature that was launched after the "checkbox yes date" (see #1). Otherwise, the default is "disabled". Either way, you can override the default by saving preferences at any time.