Any chance we could get an option in the config so that the backpack UI is off by default?

Yeah this can be added in a future update. I'm assuming you still want to allow people to enable it on a per-player basis, hence why you haven't simply revoked the permission.

This feature has been implemented in the v3.9 beta.

https://umod.org/community/backpacks/46314-backpacks-beta-390