Comment by rgovostes
Citation: https://support.apple.com/en-gb/guide/security/sec59b0b31ff/... "The Secure Enclave Processor runs an Apple-customised version of the L4 microkernel."
From the rumor mill morgue, back in 2006 there was some speculation about macOS, err, Mac OS X transitioning to L4: https://arstechnica.com/staff/2006/06/4407/
They hired a ton of UNSW graduates who had worked on L4 around that time.