Is Anonsurf unique to Parrot only?

Is Parrot the only OS that has Anonsurf?

No. AnonSurf was a fork of similar project on BackBox. But with improvements over time, i’m sure you can call some features in AnonSurf unique.

You mean unique to Parrot? Do you recall what the similar project at BackBox was called? Can I be on any debian os and download Anonsurf via command line?

Anonsurf was originally developed for Parrot but has been forked many times.
I am including two links check them out and see what works
GitHub - Und3rf10w/kali-anonsurf: A port of ParrotSec's stealth and anonsurf modules to Kali Linux” and “GitHub - baitsalem/anonsurf-debian” .
Hope it helps.

Anonsurf will only work with Debian? Will it work with Ubunutu GNOME desktop? To download Anonsurf I click:

Theoretically you could compile Anonsurf for any Debian distro, look at the source code, especially the Makefile.