It’s rightfully warning you as granting an app access to your notifications can have widespread consequences
If you don’t feel comfortable being a guinea pig and possibly having irrelevant notifications made public better not use it.
When listen submission is all you are after I’d recommend Pano Scrobbler instead which is more mature. If you are extra paranoid you could also set up a throwaway last.fm account to submit your listens, check the results and then manually import from last.fm to ListenBrainz