Uninstalling via `remove --cask --force obs` then `autoremove` and `cleanup ` still leave leftovers `/obs-studio/` and crap in `/SystemExtensions/` about homebrew-cask HOT 3 CLOSED

To remove all the files associated with a cask, you need to pass the --zap flag to brew uninstall.
e.g. brew uninstall --zap obs

Regarding the system extension, as far as I know this is a MacOS system-level limitation, and this would be the same case were you to remove the application the usual way (it is not a Homebrew-specific limitation).

from homebrew-cask.

Bypassing MacOS system-level limitation with Disabling and Enabling System Integrity Protection then solved with

• run csrutil disable in a Recovery mode terminal
• restart normal and deleting unwanted folders inside /SystemExtensions/
• run csrutil enable again in a Recovery mode terminal then restart normal

thanks to @bevanjkay for the e.g. brew uninstall --zap obs and clear explanation

from homebrew-cask.

Thanks for confirming @alemens

For anyone stumbling upon this, please read Apple's documentation about System Integrity Protection here, before disabling it.

from homebrew-cask.