I've never been bothered with meta-game concepts in the game design of any game, or with meta gaming in general. I guess it is a sliding scale, but most of the time I don't mind my players thinking as gamers. There's nothing wrong with approaching the game like a game. Whether that means using game rules that have no real in-game explanation, or thinking about the game strategically (instead of what your character would do), it's fine.
And on the latter: I've had situations in my campaign where a player wanted to prepare spells, but as a player he expected undead, while his character did not have any in-game reason to expect those. So I just said to him: Bring what ever spells you as a player want to bring. It's only a game, and it is okay to approach it as a game every now and then. I think this relaxed attitude of mine towards meta-gaming in general, explains why I feel equally relaxed about meta-game mechanics.
What about you old schoolers? There is a lot to like in some of the old school games but I find them not systematic enough for me. Heck 5e probably isn't as much as I'd like. Everything is a special class rule. I do think feats as a mechanic might be better ala Pf2e. But I am also thinking they'll make some pretty awful feats as well.
Does it really matter in regards to the metagame side of the discussion, what we call these game mechanics? Whether they be special class rules or feats, they are still special abilities that a character can unlock by leveling.
As it just so happens, I'm currently working on a semi-realistic board game, in which the game mechanics all need to be justified by realism in regards to its subject matter, while still trying to keep the game light on rules and easy to understand (and remember). So, although there is some clear balancing being done in the rules, I try to have a logical explanation for everything, that is also internally consistent. And I find that is quite a challenge. I have done away with hit points entirely, and with armor values, or tracking ammunition. This abstraction seems to work in this game's favor, because it means there is less to keep track of, and it simulates a certain degree of deadliness in combat that is in line with the realism goal. I've basically replaced hit points, with either being wounded, not wounded, or dead.