Come on! You are just picky now. You don’t need mail client that reads markdown; Discourse sends HTML via email.
Discourse is readable without javascript (you can’t post anything, but you can read). It’s half-crippled, to say so
As for disabled people: I don’t have any experience in developing ARIA sites, but looking at the source, the code has plento of aria-label
attributes (which is much more than it was on phpBB).