@buttondown Well, I wouldn't be surprised if it's that simple. Most platforms just haven't seemed to prioritize technical writing.
When I dug into the internals of emails that were actually sent by a variety of platforms, I found plenty of CSS to support other features. It didn't take much more to support highlighting.
If you don't mind my asking, did you find a few clients that don't handle it well? I don't have access to good email client testing tools.