These days you can't even get a ProtonMail account if you use Tor unless you give up a real cell# or credit card#. They only offer bitcoin payments after you identify yourself. They've become privacy-hostile themselves.

From what I can tell, you don't have to identify yourself to use bitcoin payments, but you need to have an account. So if you're just registering and want to use bitcoin to bypass their spam filter, it isn't possible and it makes no sense though...