? I know that this is a GenodeOS topic, but the GP wondered about seL4 which has some parts in GPLv2 and other BSD (2-clause).

IMHO the reason that few use seL4 is that it isn't ready: AFAIK seL4 isn't able to use efficiently multiple core with power savings, which is mandatory for usage in phones (and phones can use complex CPU with big and little cores).