Earlier quoted context omitted.
The current version of QUIC is designed to support HTTP/3, but the QUIC roadmap includes support for other protocols as well.
I thought IETF QUIC was higher-level agnostic from the beginning, has that changed?
The main difference from an API pointer of view is that there is an additional multiplexing layer. On TCP/IP the byte stream is directly on the connection. On QUIC a connection can contain multiple streams - which are openable by both sides. And each of those streams is a bidirectional or unidirectional byte streams.
QUIC doesn't care what protocol you run on top of those streams. HTTP/3 is just the only one which is specified at the moment.