It was moved from linux-staging to proper Linux in 2014 [1], and it's included with Debian oldoldstable, oldstable, and stable as well as Ubuntu linux-image packages, as a module binder_linux.ko; just need to modprobe binder.

1. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

2. https://packages.debian.org/search?suite=buster&arch=any&mod...