reslly is there a reason, even just with feats. skills, spells and class features we already RP learning things from NPC friends/mentors alot
It depends...
In our games, we learn "Class" things from NPCs all the time, but I can't ever think of giving an NPC unique ability (i.e non "Class" things) to a PC.
As pointed out, would you allow a PC to learn a medusa's gaze? I wouldn't unless it were part of a wish or otherwise intrical to the campaign.
Would I allow a PC to learn a fireball non-spell? Probably not, because the only point of it would be to prevent it from being counterspelled.
It seems to me this all comes from this NPC spells that are not spells thing. And honestly I need to see how this actually comes out before I figure out what I think of it.
I already have done things like have monsters that have abilities that are spell like but are not spells. The most recent was a death orb that is similar to a flameskull but is a magic/necrotic creation. More like a sentient artifact. And it's fireball ability did necrotic damage and wasn't a fireball or a spell.
And would I allow a PC to learn that ability? Probably not. It's something innate to the creature. Might there be other abilities that they could learn? Yes, but again, would have to be integral to the story.
Otherwise it seems more like a means to characters to just make their PCs more powerful by skirting around the rules, and that's something that I do not like or enjoy.