Apache license, apparently.
UPDATE: I just re-read the old Git thread about Shizuku’s licensing etc and their restrictions that they are trying to put on it (which I do sympathize with somewhat), so yeah it looks like it’s not technically FOSS after all, despite the license claim.
But it’s still useful for those of us who are trying to get away from rooting but nonetheless looking for some solutions from trustworthy devs to gain back a little more flexibility, as long as we’re not stubborn about everything being 100% FOSS.