I think it’s the only official one with an airport designation (B18). I’m sure there are lots of lakes that have a plowed strip out there.Only ice runway in the contiguous US? Not so.
It may be the only one that is colocated with a charted seaplane base, but it's most certainly not the only one. I know that someone puts one together on Lake Winnebago (the big one just east of OSH). And there's another thread about one up in MN on Mille Lacs too.