So it does get a bit tricky.
First off, cards like Palace Book and the Gym Badge promos, if included into an English main set, wouldn't be placed at the end of the ultra rares. FA Steven was obtained in Japan through the purchase of Emerald Break booster boxes and just looking at the XY-on main sets, only one of the cards that were released as booster box promos in Japan got put into main sets internationally (Goodra - PHF), while the other half were released as their own promos through blister packs (Noivern Pangoro, and Ditto). While we don't know the fate of FA Steven or even the Celebi-AT, it does have a shot at being included and the space definitely allows for it.
Out of all of the unreleased promo cards, the only card that fits as #98 is FA Steven, other promos would be included elsewhere in the set and those spots are accounted for already.