You're correct that they're used a decent bit in the defense industry. I can't talk too much about it for obvious reasons, but it's an area where I would expect an uptick in their usage within the next decade in the defense field alone. Check out the DoD's modernization efforts regarding seL4 and L4Re. In-depth knowledge of these microkernels is going to be a skillset in very high demand as defense contractors are forced to adhere to the modernization efforts pushed forth by the DoD.

Tangentially, I'd have loved to see a modern, (se)L4 based desktop (general purpose) operating system, capabilities-based, with services written in a safe language, with no legacy (POSIX) baggage.

Failing that, I hope to be able to hack on something along those lines when I retire :)

(before someone mentiones Fuchsia - Zircon is cool'n'all, but we're talking about L4 here)

https://kernkonzept.com/ is a company developing the L4Re OS. The company is a spinoff from the TU Dresden Operating Systems group. Afaik their main customer is the automotive industry which uses their hypervisor. L4Re is also certified for all kinds of government secrecy levels, so I would assume they have some users in that area.

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...

seL4 foundation member list[0] might give you some hints.

0. https://sel4.systems/Foundation/Membership/

You could check out Helios, which is inspired by L4 [0]

[0] https://ares-os.org/docs/helios/

Apple’s Secure Enclave Processor?

I believe the Qualcomm modems run a L4 variant.