A long time ago, I was looking for information on this weird Sonic game I remembered playing even longer ago. I remembered it was a PC game that came on a CD-ROM, and the first level (which had the same music as Flying Battery) was called "Robotropolis Zone". It also didn't work very well, at least not on the computer I was running it on. I still haven't found it, but one of the first things I did back then was search for "Robotropolis Zone", as it was the most concrete thing I remembered. The first thing I found was SRB1, which has a level with the same name, albeit spelled incorrectly.
I figured out pretty quickly that it wasn't what I was looking for, but that was how I first learned about Sonic Robo Blast. I don't know for sure if that was when I first started playing (or even found) SRB2, but I do know I first played SRB2 in 2006 (no connection to my username) when "Final Demo" was the latest version.
When I was looking for information on that game, I figured it was a fangame, given the low quality, and the fact that I'd never heard of it anywhere since. But I think it's probably some kind of bootleg, now that I know what those are.
EDIT: Decided to ask about it on Sonic Retro, and someone found it. Turns out it was in fact a fangame, though I guess it might have been sold on a CD as a bootleg, if my memory of it being on a CD is in fact accurate.
See this thread.