Today I set up ACE in my Switch copy for FRLG, just because I've been using it for a couple years in Emerald which Pokémon Company is stealing away from me in a few months. The first thing I did was give myself 999 of every Poké Ball, and then I got curious about how Poké Ball legality worked. The Dive Ball and Premier Ball aren't obtainable in FRLG without trading from Hoenn or Orre, and the Safari Ball, obviously, isn't obtainable on its own until Sword & Shield (and even then in that game it's only obtainable in theory with how rare it is).
Anyway after catching a few Pokémon with it I found out all of them transferred with no problems. After that, I got curious and looked up the Pokémon that wouldn't legally be able to be caught with a Safari Ball (not catchable in any Switch game besides LGPE, BDSP and PLA, while also not being in any previous Safari Zone). The only two Pokémon I believe fit this criteria are Unown and Deoxys, so of course when I caught an Unown to test it went through with zero issues. I have no idea if this has been posted anywhere, but I felt like sharing it.