As far as I remember this had nothing to do with FileZilla in particular and was instead SourceForge repackaging all the popular software hosted on its platform with adware.
Read beyond the first paragraph. This wasn’t a one off and they didn’t stop.
Ok but it has been going on at worst since 2018, not for a decade.
No, it has been going on for a decade because they knew about it and defended the practice. You only need to look on this very thread to find more information.
The download from filezilla-project.org even now has “bundled offers” at least for the Windows version (the Windows download page even has this disclaimer “This installer may include bundled offers. Check below for more options.”).
They must have been aware of it
Nope. They were a victim of this at one point, but also decided to do their own adware bundling. Sourceforge doesn't do this anymore.