But you can turn that behavior off, IIRC it tells you the environment variable to set if you don’t want it to do that every time it runs.

I agree it’s annoying, but I haven’t turned it off because it’s only annoying because I’m not keeping my computer (brew packages) up-to-date normally (aka, it’s my own fault).

I'd be much happier if it were on a background job, than arbitrarily running when I invoke a command

Terrible default behavior is a great reason to abandon a software package.