I really do not see how the layers matter. They are just an artificial categorization. E.g. SCTP is both a layer 4 protocol and a layer 5 protocol (SCTP-over-UDP) and as far as I can tell from some quick googling at least both Linux and FreeBSD implement both in their kernels. What matters is if various libcs add QUIC or not.

And the Linux kernel implements other layer 5 protcols like WireGuard.