As others have noted, jockstraps have not been widespread (Ha!) for decades. They started to wane in the later 80's, I think. Others have noted here that compression shorts took their place, even tho such shorts SUCK at ball support. I think the open ass of the jockstraps always been a target for gay-comments. Does anyone know why the ass is open? Some guys say it's too avoid extra sweaty "swamp ass." Myself, I just wonder if it isn't economics: less material in the manufacturing.
Certainly they been associated with gay men since their creation. you can find pics from the 1950's and before that are homoerotic, and ever more explicitly the later you go. that said, ALL the guys I went to HS with in the 70's wore jocks. We all did. It wasn't even questioned.
Like you, I went to a sports story perhaps 10 years ago and asked a young man at the counter, "Do you guys sell jockstraps?" I did the because it as a cheap thrill, I admit. The older man, possibly 40, who was this kid's boss heard this and immediately stepped in to divert me. It was clear the kid have no idea what a jockstrap was.
And yeah, I used to see 'em in stores, including men's clothing stores and plain ol' drugstores (usually sole Ace jockstraps, which sucked, but sometimes Bauer and Black, which didn't), and in 2014 I stumbled on a small independent sports store that still had a display rack of Bikes. I wanted to buy the entire display, but even I couldn't do that, so I bought 5 of them, I think. This nice girl of a certain age wrote down my order in neat handwriting on their old-fashioned carbon copy pad. I recall seeing the word "jockstrap" in her handwriting. I found myself wondering what she was thinking.
I've found two vintage REAL Bike jockstraps on eBay. I've had them for like 2 years. On one of them, the pouch threads are starting to come out and make little "loops." The other one is holding up ok.
Do not buy jocks from Bikeathletic.com. The "classic" one isn't really like the vintage ones. Also, I order sports briefs from them, but the pouch is way too small.