Yeah. Also things like filesystems. More generally, the history of BSD is
full of proprietary forks that never got merged back in: Ultrix, SunOS, BSDI's BSD/386 (later BSD/OS), Winsock, and the Wollongong TCP/IP stack on UNICOS and, I think, also on VMS. The most famous fork is macOS Darwin, which I think is still in fact open source, but it's been many years since I saw someone successfully running the open-source Darwin.
Also, though, GCC got Objective-C support, and still has it, because the FSF told NeXT it would violate the GPL for them to attempt to make Objective-C a proprietary add-on to the GCC compiler, even if it wasn't literally linked with it. And a lot of GCC backends probably would have been kept proprietary by one or another hardware company if the license had allowed it.