In T277638: Separate user setting for RefPreviews and Page Previews for default we introduced separate settings for the two popup types Page-Previews and Reference Previews. Before, ReferencePreviews was a BetaFeature the user can opt-in to, to be stored in the database with the key popupsreferencepreviews. The current code reuses the same key when not in beta any more.
While there is code in place that tries to not persist the beta feature flag as long as the user doesn't intentionally interact with it, this doesn't work. The moment a user touches any user preference – no matter which one – and clicks "save", popupsreferencepreviews is stored with the default 0. We want these users to get ReferencePreviews. Since there is no way to distinguish this (large) group of users from the (small) group of users that enabled and later intentionally disabled the beta feature, we need to enable the feature for all of them. This can easily be achieved by using two different keys for the beta and the non-beta preference.
This was announced like this: