[MarkusWandel]

For what it's worth, my SD card is unencrypted so Android treats it as external storage, rather than encrypted and integrated into Android's "system" storage.

Anyway what's left of filesystem access for Termux isn't really a blacklist as in "you're forbidden to go here" but rather feels like a whitelist. There are a few carefully selected places you're still allowed to go, front and center being the DCIM directory.