looks like rendering adblockers extensions obsolete with manifest-v3 was not enough so now they try to implement DRM into the browser giving the ability to any website to refuse traffic to you if you don't run a complaint browser ( cough...firefox )
here is an article in hacker news since i'm sure they can explain this to you better than i.
and also some github docs
tuta has had desktop clients since the end of 2018. https://tutanota.com/blog/posts/desktop-clients/ if you're interested
if i can't use it in my already existing mail client it doesn't exist to me
This. I'm much interested in Protonmail and Tuta, but until they can support SMTP / IMAP or have a publicly accessible bridge to operate with client-side mail, they're absolute nonstarters for me.