I'm not an expert on the GDPR (and I'm frankly sick of hearing about it) but surely what it bans is tracking cookies, and not cookies necessary for the proper function of the website?
Exactly, it regulates collection of PII, which includes most tracking cookies.
It doesn't really regulate "refusing service due to disabling of browser features", but then again it's a complex law, so someone (even a court) might argue otherwise, but it's doubtful.
I am in the EU