Darwin is still open sourced, but mainly because of drivers and differences in certain other things (threads maybe, Mach ports / IPC, etc) that don’t have the integration or implementation with existing software is a major headwind to attempting a full encompassing Darwin OS.

PureDarwin was (is?) an attempt that hasn’t been able to break through those headwinds.

Is Darwin used for something other than research? I'd really love to see some homebrew stuff with this kernel.

There's https://www.puredarwin.org/ which is probably great for partial OSX/iOS emulation efforts. That's it as far as I can tell, though.