This is the way Termux works so far, External storage: Storage on external SD cards. Each app has a private folder on the external SD card, and interchange between them needs to use a special API not yet available in Termux.
It worked fine in Android 7, when I updated to 8 my scripts started getting permission-denied errors on writes.
I don't see anything in that link that says that sdcard (outside private folder) is read-only.
The way I read "Each app has a private folder on the external SD card, and interchange between them needs to use a special API not yet available in Termux." is that I can't read another applications private folder, but I'm not trying to - I just want (write+delete) access to the publicly accessible parts of the sdcard (lots of other applications do this, and termux did in Android 7).
Well I have Android 7 and termux works in the way you described at first: I have write access to the private directory for termux on the sdcard but nothing else... other applications can access every DIR in sdclard due implements SAF: https://developer.android.com/guide/topics/providers/documen... and I think this what termux doc refers to.
https://wiki.termux.com/wiki/Internal_and_external_storage