And nobody learned their lesson and they all piled over to the next centralized system that offered "FREE!".
And so it goes.
Edit: 2001, I see one (1) banner ad, and that ad was seemingly for an OSDN (Open Source Developer Network) conference. https://web.archive.org/web/20010517002942/http://sourceforg...
Given SourceForge only hosted Open Source software, and had no source of revenue beyond ads and sponsors for quite a long time, AFAIR, I think they get a pass on a banner ad.
(My SF account dates from June 2004. I expect I was thinking about using it as version control for a FOSS project I was working on at the time, though I don't know why, as it seems SF didn't support svn until 2005. Maybe I couldn't find any better options? The pre-GitHub ecosystem was pretty bad! But, luckily, I ended up not having time for any FOSS stuff from about autumn 2004, so: problem solved. And when I next looked, in early 2010, everything seemed to be git+github, and all the better for it.)
But, it had a few years where every OSS developer I knew had nothing but positive feelings toward SourceForge. It gave one of the projects I work on thousands of dollars worth of transit over the years. It's hard for folks who've only ever worked on an "everything for small developers is a loss leader" internet to understand that we used to pay for and manage our own servers. I had a $200/month bill for just my Open Source projects on a couple of colocated servers.
Yes, SourceForge went through a lot of shitty stuff. The overtly hostile stuff (adware inserted in OSS projects) happened after it changed hands. But, when the revenue of their original model dried up and they couldn't stay on top of new development (being slow to offer a good git experience was a fatal mistake).
Anyway, it's not great now (though it is now owned by seemingly decent folks, who haven't really been able to find a way to make it work), and it went through a period where it was a borderline criminal enterprise, but it started out as a genuinely helpful part of the OSS community.