xxd is distributed with Vim.
I've never thought of xxd as being anything but a "Vim thing".
That it had its own history (1990-1998) before being pulled into Vim is a TIL for me.
Here you go:
https://github.com/vim/vim/tree/master/src/xxd
There might still be other versions of this out there, but I suspect most OS distros pick up xxd by way of packaging Vim.
So if you work on some other xxd, you're probably in a dead fork.
The man page is also from Vim:
https://github.com/vim/vim/blob/master/runtime/doc/xxd.1
My first patch to xxd, if I were to work on it, would be to fix the man page to indicate clearly that it's the xxd bundled with Vim.