I found strange that no one mentioned Privnote (https://privnote.com https://privnote.com), a much older/popular service that does this but more securely since

I found strange that no one mentioned Privnote (https://privnote.com https://privnote.com), a much older/popular service that does this but more securely since it generates URLs with keys in the fragment to encrypt the messages on client side (so that the plain message never hits the server).