I hear it is not legal to use 209 shotgun primers in muzzleloader hunts in Idaho. Is that true?

What you heard is correct; 209 primers are not allowed for muzzleloader only hunts in Idaho. The primers may be used in short-range weapons hunt. But a number of other features available on some models of muzzleloading firearms are not permitted. Before purchasing or using a muzzleloader in an Idaho big game hunt, the current big game brochure should be consulted: see page 12.