But the fee that the GEZ used to collect (the Rundfunkbeitrag) does still exist and that's what a lot of people call it even the entity that collects it has a different name now, which doesn't make a difference to its effect.

I believe there are taxes in Germany that are zweckgebunden (legally bound to be used for a particular purpose), but the Soli isn't one: there were political justifications for its introduction, but it goes into the general fund and the government doesn't have to use it for those purposes.