It's such a bizarrely specific method, and I never saw any hinting towards in SwSh, so it makes me wonder if they're aware of sites like Serebii and expected players to look it up if they wanted a Runerigus of their own.
As you mentioned, there is absolutely zero in-game text or hints to suggest how Yamask evolves. No mention that it favors a certain location, that it might need to take damage, nothing. It's not even like Farfetch'd's evolution that you might induce through sheer luck or chance, there is simply no way you can accidentally trigger Yamask's evolution without knowing exactly what to do. I suppose it's possible to happen by complete chance, similar to Farfetch'd, but given how specific the damage requirement is, I kind of doubt it. I really would love Game Freak to explain how players are supposed to figure out certain evolution methods using absolutely nothing but in-game text. I suppose most things are possible through trial and error, but then it's basically devolving into the infinite monkey theorem.
So no, there is nowhere in-game where it is mentioned. Source: https://gamefaqs.gamespot.com/boards/259372-pokemon-sword/78201642
