I’ve linked a pretty huge github issue below, but the tl;dr is this: Native code which is not packaged in an .apk may not be executed by the user or non-system applications.
The solution they are currently employing is to repackage everything as .apks, which will be downloaded and installed by the pkg utility in termux. Many applications will still work, including interpreted languages like Python or Ruby. The same cannot be said for compiled languages.
One proposal is to compile code as an apk and installed by the user, but for a language like Julia with as much metaprogramming as it contains may not be compatible with that model.
https://github.com/termux/termux-app/issues/1072#issuecomment-474385386