The key can be found by searching codesearch. I guess the result is everything one can add the the configuration parameter.
Topic on Extension talk:BetaFeatures
Sorry, I don't understand your question.
This is actually not a question. It is a note/reminder. Since you oppose to keep a list of settings to this configuration parameter and I am sick of searching my b.... off every time I look for them I figured that just adding this as a note/reminder is the best we can do.
OK. But your assertion is wrong. Not everything uses the key only for the Beta Feature preference to see if it's enabled – see for example VisualEditor. If you really need to know, look at array keys returned on the GetBetaPreferences hook for values. But, again, you absolutely should not ever use this.
I guess this is just a complete mess. I seek for a list of these keys. So how do I do this?
Don't? This is a bespoke, one-off feature for me personally to manage as a last-gasp security wrapper around Wikimedia production.
I did not understand your answer. The issue I am trying to solve is hiding features which do not make any sense for isolated instances, e.g. "Compact language links" are things which are more or less senseless to offer to users in such environments.