Let's get the canonicity issue right, shall we?
I believe the only place where regenerative shields are mentioned on screen is in VOY "Message in a Bottle", as one of the tactical systems aboard the experimental Prometheus
. They are not described there - and they are not specified as being experimental or otherwise new, either. They are part of a list that is given by the ship's computer and rudely interrupted in mid-run by the EMH:
"Primary battle systems include regenerative shielding, ablative hull armour, multi-vector assault mode-"
We don't know if those things are mundane or exotic. The MVAM is a novelty, and the EMH immediately queries on it. But the EMH does not query on either regenerative shielding or ablative armor, so both might be known and common quantities aboard all or most starships. And the list doesn't seem to go from more exotic to more common, judging by the way the EMH's interruption is timed - and is unlikely to go from more common to more exotic, either, because then things like phasers and torpedoes should have come first. Essentially, we're looking at a random order, then.
The TNG Tech Manual would have us believe that all starships and specifically the E-D have ablative armor to some degree. DS9 dialogue, OTOH, would have us think that the ablative armor on the Defiant
was news to Captain Benteen. Yet perhaps it was merely the amount of armor on that ship? All warships today are armored, in the sense of having metal or at least glassfiber plates against the elements. Yet very few carry so much of that metal or other material that it would make a difference in battle. Perhaps it's the same deal with (ablative) armor and (regenerative) shields - every ship has those, but the balance is different in different ships?