Don't most apps allow you to paste plain text with Ctrl+Shift+V nowadays? That's what I almost always use, unless I have some use for the copied rich text.
Annoyingly the GitHub comment box doesn't. Incredibly frustrating to copy someones name from the GitHub webpage and paste it and it pastes as a markdown link which doesn't trigger a mention.
Aside from GitHub doing something dumb here, Paste without formatting belongs as a browser or OS feature, not per-website code.