I think metroidvania is probably a genre reserved for titles that directly mimic the structure of Metroid, Castlevania or something in between. I don't think RPG elements are a requirement, so in that sense I think that metroid-like is probably more fitting for many of them, but metroidvania just allows the term to cover a wider scope of games, rather than having two terms (metroid-like and castlevania-like).
As for using metroidvania as an adjective, I don't think that actually has a lot of value. I think some degree of 'metroidvania' design in open world games is almost a given today. A large number of games are driven by ability acquisition, with players motivated to keep playing by the idea that they're progressing, and growing stronger by playing. Therefore, one of the means in which you make the player feel like that progression is worthwhile, is to hint towards it earlier on in the game. You see it in everything from the Batman Arkham, and God of War games to Spyro Reignited and Pokemon.
I don't think it would really help anyone understand what Spyro 2 or 3 is doing if I were to use the term metroidvania, or Pokemon for that matter. I think we have better terms for describing that element of the design that more people will understand.
Spyro 2 is a 3D platformer where you collect gems and orbs in order to progress through levels and learn new abilities.
Spyro 2 is a 3D platformer where you progress through a series of levels with metroidvania elements.
I think just saying that you learn new abilities fine. The player can presume that the abilities are used for something valuable, such exploring more levels, because that's the predominant nature of the game. Describing it as having metroidvania elements wouldn't help many people understand what's going on, because a lot of people aren't familiar with the term, and it's quite a minor part of the game, really.
I think for game designers, for discussions on forums on ERA and whatnot, metroidvania works fine as an adjective as it describes a specific type of gameplay loop where the acquisition of abilities is allowing players to probe deeper and deeper into the world. In that sense it's like a game design pattern, or a term that encompasses a specific set of game design patterns that might help people who are well versed with game design nomenclature understand the type of design that your game is trying to convey.
But ultimately if I used the term with my mum, or even my sister (who both actively play games), they wouldn't have a clue what I was talking about. I'd describe Ori and the Blind Forest to them as a 2D platformer and metroidvania, but I would never use the term as an adjective to describe any gameplay elements with them, just because it doesn't mean anything.
