mrpingouin1 they could do it like they did with JSON_Signs, whenever you put a Spawnegg out of your creative inventory tab it removes the EntityTag {same for ArmorStands}
Once they fix this bug, You can make a spawn egg that has an iron golem (Or any other mob with no spawn egg) as the passenger and its riding a mob with 0 hp. (The mob dies instantly and you keep the passenger, essentially allowing spawn eggs to spawn anything!)
can confirm 1.19.1 I adapted an invisible Allay mounted on a wolf so that the "dog" has two new behaviors: atack and collect item (even if it walks sitting, but can fix with a rope), but it doesn't work when it is applied to an egg to be sold by the villager as a "pet Shop".
Confirmed.