Since the Beta Features extensions also registers features from extensions that are not installed for that wiki it will be good to list also the respective feature strings here, e.g. tmh-videojs for the new video player to be used with the $wgBetaFeaturesWhitelist. This would allow to easily identify and exclude these unavailable features.
Topic on Talk:Beta Features
I don't think that's a good idea. The list lives in config as wgBetaFeaturesWhitelist and no-one will ever remember to update an orphaned list documenting things here when there's no process reason to do that.
Fair enough though will be more difficult to find these flags.
(Also, what you wrote is wrong; the Beta Feature extension doesn't know anything about an individual features, it's those extensions which are installed on the wiki which register themselves as Beta Features.)
A wiki I control offers Popups, HoverCards or what it is called in the preferences. Trust me that neither the extensions TextExtracts, PageImages or Popups are installed or invoked for the this particular wiki. So I wonder where this comes from. Must be a bug from core in this case.
Ah, ok. I got confused. Sorry. It is shown in the "Appearance" tab and not in the "Beta Features" tab. So this is a MediaWiki core issue offering features that are not installed.
Known-to-code-search beta feature registrations: https://codesearch.wmflabs.org/search/?q=GetBetaFeaturePreferences&i=nope&files=extension%5C.json&repos=. As you can see, MW itself doesn't register against the hook (and any attempt to do so would be reverted, as it mixes concerns the wrong way around).
Do you mean "Reference Previews"? That's provided by the Popups extension, yes, and if you can tell me the bug I'll be glad to fix it. :-)
As noted in the post you reply to this is no longer about Beta Features. Sorry for the noise.
The preference section allows users to select: Reading preferences - Page previews (get quick previews of a topic while reading a page). No idea what this could be. / Just checked for the system message keys which start with "popups-prefs-..." which I assume refers to functionality provided by the Popups extension. This one is however neither invoked nor installed. It is a core bug in this case. I will create a task for it on phabricator.