for starters. The fact that i had to google it to figure out what it was. Let alone randomly discover that it exists in a tangentially related search. It is an ASTRONOMICALLY inaccessible feature to someone who isn't readily invested into android.
secondarily, it should be done in the gallery, obviously. That just MAKES sense. The place where you are shown pictures, should also be the place where it lets you ignore more pictures. If you want to use .nomedia as a backend for that? Fine, Document it at atleast.
It's also just, weird... The gallery app only seems to consider a few folders existing at any state. Some better than others, i have no idea what drives the logic behind it. But you can nest them, super easily, which definitely won't cause any issues. If you have a single folder you do want to show, but 9 that you don't, you need 9 no media files, because that's convenient apparently.
I mean really any other system would've been better, a directory list, a file table, a database, literally anything that lets you mark it interactively. Having a single HIDDEN file, determine the state of an entirely independent app is just next level hackery. You really shouldn't ever do that. It's just fundamentally bad design philosophy. It'd be like a lightswitch on the opposite side of your home, preventing your garage door from opening.