There is also SCTP at layer 4 but it doesn't get much use outside certain domains, so certain OSes have deprecated it.
Also as others pointed out applications will just use a library as they do now with OpenSSL.
Also as others pointed out applications will just use a library as they do now with OpenSSL.
That's not because of ossification, it's because of middleware boxes like NATs and firewalls that will refuse to pass anything that isn't TCP, UDP, or ICMP.
That's how DataChannels in WebRTC were specified a while ago, and it's the only mechanism to do browser to browser communication.