THey could also just throw it in the level up move pool like they did with other event movesEruption Heatran does actually have a pretty easy solution here, and that’s just to make the move teachable by TM or Move Tutor in any game where Heatran exists. (Leave it to the diehard Guardian Signs fan to use that as my go-to example most of the time.) The same ideology could work for a few others, such as what happened with Dark Pulse Deoxys, a Pokémon initially only available via event until Generation 6 from what I remember. Unfortunately, this isn’t a perfect solution since most event exclusive moves are stuff like Psycho Boost or V-Create that most likely won’t be a TM or Move Tutor move anytime soon.
Edit: I just looked it up, and Dark Pulse was a move tutor move in Black & White 2. Sorry about that.
My point about the last example is not "there's no solution", it is "they just don't want to"