I am looking for another muzzleloader rifle and I see that most manufacturers are making rifles that take number 209 primers rather than caps. Is it true that these primers are not legal in Idaho?

True, in muzzleloader-only and traditional muzzleloader seasons, Idaho rules on muzzleloaders require that they are "equipped only with a flint or percussion cap." If you are hunting in an any-weapon season with a muzzleloader, there is no restriction on primers or caps.