Well, I don't usually follow this stuff, but I have noticed some things at the pool hall. The other kids have gizmos and whiz banger attachments.
I see butt extensions, joint extensions, and joint weights. So I looked on the web and sure enough, they are all for sale "mainstream".
What surprised me was that the OP said " it's almost impossible to play without one". I found that statement striking. What changed?
I don't keep up with the organized sport anymore, when I watch videos they are from the last century. So I looked at what is going on. I saw one article where a promoter uses fake smoke machines so the tournament looks smoky on camera, to create the expected atmosphere.
They have shirts available that look like they came off a pro bicycle rider. Some kind of "high performance" spandex-like material. I went and looked at Jayson Shaw's high run and he was wearing one. He beat Mosconi, and Schmidt, by a lot. Jason had the shirt, but didn't use an extension. Was it the shirt? Of course, Mosconi's run still stands as an actual competition run.
While Shaw famously popularized playing with a permanent 7-to-10-inch rear extension for years to gain added stability, weight, and reach, he actually removed it before his major straight pool milestones. Was this a stunt? A daredevil maneuver? Like- "Don't try this at home kids."
So, further investigation reveals there is special underwear for pool now as well. And shoes.
Mosconi set his record in actual competition with a Rambo cue. No extension. No spandex. I have no idea what kind of underwear he was wearing. He was wearing black leather dress shoes.
So, these are coming back? Well, except this is made of wood.
On a side note, a few years ago a dude came up to me at the pool hall. He said another dude said to ask me because I knew a lot about cues. He said he wanted to buy a mid extension for his cue but he didn't know how to make sure it matched the taper of his cue. Not kidding.
I have no extension. I might be a daredevil.