I stumbled upon multiple references (not sure what the original is) to a very specific error in @termux about not working repositories and switching to my termux mirror at https://termux.mentality.rip was suggested as a solution.
The real root cause of their problems is that they're using an old legacy version from Google Play that stopped updating in 2020. Not only that, since 2020, Termux repositories were renamed from <repo>-packages-24 to termux-<repo>.
The reason why switching to my mirror works because some time ago, for my own purposes, I symlinked new termux-<repo> naming to old <repo>-packages-24, and completely forgot about it. Because of this, it's somewhat possible to bootstrap from Google Play version and upgrade to latest packages.
Now I'm wondering, should I just remove these symlinks. After all, Google Play version is deprecated, it hasn't been updated for 4 years, and users should move to F-Droid version. But also there are some people (probably not enough tech-savvy to know about F-Droid) who are unknowingly relying on this.
I don't log repo access, but just by looking at HTTP error log, I can see somebody trying to get packages by old repository names.