ISA modems are still very common and quite useful; especially in Linux boxes. As to the issue of it being old, that is true, but is not a thing of the past. I've seen some Athlon boards out there with an ISA slot. Presumably, they put it there just for that purpose.
PCI hardware is the best way to go with a new computer though. We'll start seeing a lot less mobo manufacturers making boards with ISA slots. Plus I think most hardware PCI modems support Linux as well.
<font color=red> If you design software that is fool-proof, only a fool will want to use it. </font color=red>