What is appropriate attire for halter classes at a breed show? I've been told jeans are acceptable for halter. Haven't shown in years but back when I did, we wore slacks and equestrian/horsemanship jackets or vests.
Slacks, belt and buckle, western hat, boots, and a shirt and blazer or for a woman a tunic length top. At a local level show jeans (western style, dark colored) and a long sleeved western style shirt would be acceptable.
For two-judge shows, you can definitely show in more casual clothing. You can wear a straw hat, oxford type shirt, jeans, and boots. To give the absolutely best impression of your professionalism, make sure your hat is shaped correctly and clean, your shirt is pressed, your jeans are long enough/dark wash/no sparkles on the pockets/starched with the crease down the front, and your boots are in great condition.
When you get to the 4 judge or more PORs and Zone shows, then you would be better off with more formal clothing. I had a trainer/judge tell me that you can't go wrong dressing like a judge--and I have found that to be true. This means good bootcut khakis, a button up shirt, a blazer, a nice necklace or scarf, a felt hat, and good boots would be a safe option. Another good one is a business suit in a neutral color, with a button up shirt and neck scarf.
I only wear a showmanship outfit if I'm also going in showmanship.
I always recommend people go look at the show proofs on KC Montgomery's website. You can see what everyone is wearing at the major AQHA and APHA shows this year.