In #1 (comment) @agnostic-apollo suggested some additional improvements. Lets track it in this issue so it is not forgotten:

Another thing I want implemented is a blacklisted mirrors list file that is hosted on fosshost and as fallback on github as well in case its inaccessible, and file possibly signed with our key. This should contain all the mirrors that must not be used or whose domains have been freed or url sub path changed, like bintray, packages.termux.org, kcubeterm.me, recent one, etc. This file should likely automatically be downloaded on termux app start and incrementally as well to $PREFIX/etc/apt/blacklisted-mirrors.list file. The mirrors in these files should be checked by termux-change-repo and excluded from showing and pkg should exclude it too from mirrors group and switch to default mirror if none remain or single mirror is blacklisted.

An additional default mirror site file should also be hosted, that is used by the app to replace any blacklisted mirror from existing apt sources list files so that if user uses apt instead of pkg, then blacklisted mirrors don't get used. The default mirror should also be used by pkg script as default.

With the above, the blacklisted mirrors will be independent of whether users updates termux-tools or the app or has old bootstrap and we hopefully won't get any (or lower) broken mirror issues anymore. Hopefully, fosshost and github will be accessible in all countries. There is of course an ethical issue of pinging either fossthost or github continuously that could be used to track termux users, so users should explicitly opt in with a prompt. The user should also be able to disable this in termux app settings.

We could also download the files in pkg script instead so that app doesn't need to and it doesn't ping in background and only when users wants to update, but then users that may be running apt directly would still remain affected, unless we change relevant apt binaries to a wrapper scripts or patch them to run a script at start, depending on command install/update/upgrade/search.