Comment by kev009
I believe Apple uses it in at least one of the auxiliary service processors in their current portfolio under the moniker "Darbat" or "L4/xnu"
https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&d...
https://microkerneldude.org/2016/04/14/so-the-fbi-cracked-th...
Darbat was their project mainly to run macOS on L4. AFAIK this was canceled, and no work has happened since around 2006.
The newer L4 stuff (like Apple's Secure Enclave) is based on L4Ka::Pistachio, but with proprietary changes made by Apple. [1]
[1] https://www.blackhat.com/docs/us-16/materials/us-16-Mandt-De...