There was one team fairly recently that thought they had developed one that got a lot of press, but it turned out to not be true.

But that was only for that one specific case, it didn’t prove that room temperature superconductors can’t exist in general, there are still other teams working on developing them, and theoretically they could be possible, we just haven’t quite worked out what materials will exhibit superconductivity at room temperature, under what circumstances, and how to make them.

And we have some materials that come pretty damn close, Lanthanum decahydride can exhibit superconductivity at temperatures just a few degrees colder than some home freezers can manage (although at very high pressures)