We discussed something similar
in this thread several years ago, Moto Sekai Ichi’i no Sub-chara Ikusei Nikki. You even said that the methods behind acquiring skills were kept secret.
Oh, bother.
Yeah, that's the story's title. I remember now.
Disgusted, but not surprised
Villainess Level 99 has the ‘incredibly dangerous’ bit as well — the protagonist doesn’t carry a common protective amulet that results in less XP, something everything else thinks is a terrible idea. (It also has a lot of the overpowered protagonist being played for laughs or making new problems for herself.)
That's a shame to hear about the Grimgar books dipping in quality, though I've also heard that (especially since it seems like there's more than one alternate world involved, which seems like a bad idea from the get-go.) Yume is fantastic though.
It's been fun.
Another bit from that story is that the exploits ARE known in universe.
And they are jealously guarded secrets because they offer so much power.
Disgusted, but not surprised