Currently, for campaigns using the impression diet feature to limit banner impressions, we always show a banner if the user doesn't have the ability to store impression counts and related data in their browser (i.e., for campaigns using cookies, cookies are disabled or otherwise don't work, and for campaigns using LocalStorage, LocalStorage is not available).
However, it seems that in such cases, it would be better to always hide the banner.
(Note: Another possibility would be to act as if such users were always on their first impression, and do whatever would normally be done in that situation. That's what the on-wiki JS that was previously used for this does.)
We came across this when following up on the fix for T119348.
This task considers the general fallback action for lack of cookies or LocalStorage (whichever applies). Yet more options: we could also have different fallback actions, or a config option to vary this on a campaign-by-campaign basis.