I actually checked this a few months ago when I was planning a quick weekend in Boise. Southwest does fly into BOI, and I booked it pretty easily. I had to change my return flight last minute because of work, and honestly the quickest way to fix it was just calling ☎️ — they handled the switch in a few minutes. Online worked too, but the phone rep saved me the hassle. If you're planning a Boise trip, their schedule is usually decent, especially for short hops from nearby hubs.