There are basically 4 categories of rare moves I think.
i) TMs or tutor moves from old games that never got incorporated into a Pokémon’s moveset once the TM or tutor was discontinued (with VC some of these became obtainable in modern gens… with 3DS eShop shutting down, not for much longer for people who don’t already have them on VC!) The main example would be most things with Teleport for TM, or lots of things with Body Slam for tutor as the last time that was a tutor move was in Emerald or XD.
ii) Egg moves that have a very small number of parents, often requiring trading mons around before breeding. Occasionally you also have Pokémon who have an egg move programmed into them that are completely unobtainable due to egg groups, like Nosepass and Head Smash in I think Gen VI, not sure on the gen, who had no legitimate parents who could learn Head Smash themselves.
iii) Lots of Shadow Pokémon received moves only in Pokémon XD: Gales of Darkness upon purification. These can still be traded all the way up to modern generations and can be used on competitive movesets, but it requires a lot of effort. Notably I think I’m right in saying this was how you got Heal Bell on Chansey at least for a few gens. It’s definitely how you got Heal Bell on Mightyena, and Mightyena did sometimes use that in the bottom tiers.
iv) Any other move that was in a Pokémon’s learnset briefly and then taken out of it. If you trade a legal Pokémon with the move it can still be used when it’s no longer in the moveset. Your events are a good option, but another option I have off-hand is Water Gun on Phanpy and Donphan, which they could learn as an egg move in Gen II and then never again. While the Gen II event is notable, Hold Hands / Celebrate are Gen VI and VII event exclusive moves, which were very relevant during the era of Z-Moves.
I don’t think there is a compiled list but splitting it into these categories should make researching it more focussed!
