e.g.?
Discussion
Fuck I had a long reply, focused off amethyst and the draft was lost.
TLDR; PGP was interoperable with anyone at any domain, as long as you both knew how to use PGP. It was a usability disaster and now the keyservers are in shambles. It's dead.
But the replacement solutions have no interoperability. If you want easy private comms over Tuta, the recipient has to be on Tuta. Yes, Tuta has some silly webform solutions, but it is a WEBFORM, which is highly trustful.