Altimeter-based fusing is primarily used with bombs, which I tend to distinguish from warheads. Most nuclear warheads are air-burst, so it may be that some of them used altimeter-based fusing. I think they're all radar based now, but I can't confirm that.
In my mind at least, a bomb isn't the same as a warhead, though they both explode. I consider a warhead to always be a part of a weapon, with it's own delivery system. -- ansible