Pokemon Showdown allows the use of a few special event moves that are illegal with previous evolution exclusive level up moves. These include most of Pikachu's event only attacks (
Surf, Heart Stamp, Endeavor etc) with Pichu's various exclusive level up moves (
Nasty Plot, Sweet Kiss, Charm). The only other Pokemon with event + pre evo exclusive level up attack problems I can find are Vivillion (Hold Hands + Iron Defense, String Shot, Tackle, Harden is not caught as an illegal combination) and
Altaria (False Swipe + Mirror Move + Cloud Nine; False Swipe + Mirror Move is legal thanks to a 3rd gen event).
As an aside, Endeavor + Pichu Level up is compatible, but only from
this event meaning a shiny + jolly nature lock akin to the Shiny Johto Legends should be in place. Note that there's a non-shiny Endeavor Pikachu so that enforcement is only required for Pikachu / Raichu with Pichu level up moves.
Edit: On the subject of Pikachu stuff, Surf + Sing or Petal Dance or Dizzy Punch Pikachu and Raichu should be legal in Gen 2 and Gen 1 tradeback rule sets. Sing comes from the Pokemon Center events from a Pichu egg. You can then evolve the Pichu into Pikachu, transfer to Gen 1, beat the Prime Cup in Stadium 1, and tutor surf. Shouldn't be any legality issues with it, but the team builder is marking it as illgeal (unless I'm missing something).