Thought I'd come back to this thread as there are some...issues with the hackcheck that make this deceptively complicated to add.
Beelzemon 2003 bought this up before in the Gen 1 PS Dev Thread, but it's just not been looked at properly.
I'll be blunt here: Gen 1's hackcheck is
awful in regards to levels. It hardly even considers them, and I'm not really gonna fault it, as RBY is usually played with L100 Pokemon anyway. Here's an example with an L2 Onix with a TM-ridden moveset. I used this for a mechanics test elsewhere, just changed the level and...boom. The lowest possible level Onix can be obtained in RBY is L9, I believe, but I could be wrong.
And trust me, this is hardly the only case where this happens. You can quite literally use an L1 Snorlax, despite L1 being impossible even on a technical level! The lowest possible level a Pokemon can be obtained is L2 Rattatas or Pidgeys just outside of Pallet Town. There are also other pressing issues here, such as Yellow changing the minimum levels for a bunch of Pokemon through in-game trades or wild data changes. I think Blue JP may have some changes too, but I could well be wrong.
You may ask: "Plague, does this affect the level-up learnsets?", and I have some answers on that. It actually takes the level-up learnsets well, though it doesn't really explain why your Pokemon can't get the move. Let's put Bind on this Onix and see what happens. Then, I fix the level by making Onix L15, which is when it learns Bind.
Bit vague and misleading, innit?
A quick lookup on Bulbapedia explains the issue: Onix can't learn Bind until L15. It's not that it can't learn Bind, like PS kind of says. I'm pretty sure this has caused issues elsewhere, but eh, not why I'm here.
RBY 2k20 is starting to add OM Rulesets (including Pika Cup actually) with varying levels to their PS Server, so naturally, this came up. It's a long-standing issue that would definitely complicate the addition of the format there, and on PS Main if it's ever considered.
TL;DR The RBY Hackcheck is a dumpster fire in regards to Pokemon levels and will probably need fixing if this format is ever even considered for PS Main implementation.