The one removed here is an implementation flag, we use them when we develop features to disable them in case they break the release. Once a feature is released with no breakage, these prefs are removed. These are prefs only for an implementation detail, they were never intended to be user prefs. Bug 1629303 is the bug to add a user pref.
Bug 1627989 Comment 8 Edit History
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The one removed here is an implementation flag, we use them when we develop features to disable them in case they break the release. Once a feature is released with no breakage, these prefs are removed. These are prefs only for an implementation detail, they were never intended to be user prefs. bug 1627858 is the one to add a user pref.