That's a hard problem that a lot of smart people are trying to figure out. I'd say there's been good progress.

https://support.google.com/messages/answer/10252671?hl=en

I think this is exclusive to Jibe and Google Messages but a widespread (in terms of installs) proof-of-concept is still pretty cool, and better than nothing.

The bigger problem is adding E2E to SMS is going to attract the FBI's ire, they really really really like reading everybody's texts.