This is really confusing. There's this ticket T235083: Full Beta for the ReferencePreviews feature but instead of just enabling the feature everywhere as a beta feature by letting the defaults[1] do their thing, we seem to still have "limited" the "full beta" to the wikis where the Popups extension is available. And I'm not sure why,

IMO it makes sense to default wikis that enable page previews also to enable reference previews at least as beta feature. - And in the future to default these wikis to having the feature enabled as well.

[1]

$wgPopupsReferencePreviews = true;
$wgPopupsReferencePreviewsBetaFeature = true;

Postponed till we do a general cleanup after full rollout.

As far as I can tell, these settings don't do anything, ReferencePreviews is available everywhere as a beta feature.

That's not entirely true anymore. Back then the combination of wgPopupsReferencePreviews and wmgPopupsReferencePreviews wikis was matching the wikis that generally have the Popups extension available with wmgUsePopups. Intentionally or not, that differs currently, because more wikis have enabled Popups now. - Also the setting itself does make a difference and can kill the feature on a wiki wants page previews without reference previews.

So I guess we're back to the question, if it makes sense to have that feature switch. - I'll update the ticket accordingly.