Jochen Liedtke became a professor in 1999 in Karlsruhe, sadly he passed away only shortly after in 2001. I don't know if his successor Bellosa still does research on L4. There was the L4Ka project which appears to be completed. In the bachelor lecture on OS by him it's not part of the curriculum.
Rittinghaus, alumni of Bellosa, is involved with Unikraft [0], which was featured a couple of times on hn, and is using unikernel technology.
After Liedtke's passing, L4 research was continued in groups at UNSW (Gernot Heiser, formal verification -> SeL4) and TU Dresden (Hermann Haertig, Fiasco/L4Re, focusing on real-time and secure systems).
Genode (already mentioned in another comment) [1] came out of the TU Dresden group with some nice ideas around managing compartmentalized systems. Kernkonzept [2] is a startup commercializing the L4Re microkernel.
Rittinghaus, alumni of Bellosa, is involved with Unikraft [0], which was featured a couple of times on hn, and is using unikernel technology.
[0] https://unikraft.org/