Well, an email client isn't a tool just for developing - it's a generic tool that has a lot of use cases. I don't think one should sacrifice features to get diff highlights or filtering PRs, nor should a generic email client try to implement out-of-the-box SourceHut filtering functionality.

Yes, one can use specific filters to accomplish subscribing to a particular PR, but that's complex, and when I'm using a generic email client I can't think of how it can not be complex.