It isn't that email is taboo. It is that it is effort.

I have to go to my email program. Download the patch. Upload it. And then, if I want some changes, have to copy-and-paste stuff into a reply. Then repeat.

That's more effort than clicking "review" on a hosted git service.

A theoretical future service could use email under the hood to send messages back-and-forth between instances. But that's just an implementation detail.

That sounds like a UX/DX problem, doesn't it? You could have a tool that automatically deals with the tedious/repetitive parts of the workflow.