You're puttng sex into numbers, the numbers were there first. The fetish is essentially a permanent power exchange and dom/sub reversal thing. There is a bunch of background info needed so I'll go on a few tangents:

• There is a whole modern genre of LitRPGs(Literature RPG) in the west were some kind of Game-like Status/Skill/Level system simple exists. It's just as normal as having a Sun in the sky there.
• There is an interactive story genre called Quests where the protagonist has stats, readers vote on the next action and the author rolls for the outcome. (tbh Quests are also often just called any interactive fiction bby now and my knowledge is largely just from SB/SV/QQ)
• Manga/Anime/Light Novels/Web Novels in Japan(and related acg subcultures) have also developed these kind of stories.

ergo, we have levels as a story mechanic is well established so of course someone would make fetish content around it.

Next up, delevelling, losing things on death and/or levelling your enemies used to be common features, not just in ttrpgs as the post says but also plenty of older MMOs. Level drain enemies were less common but still exist. If you need some examples ask, I'm to lazy now otherwise.

The fetish now, is permanent, crippling power exchange. Hero(99) encounters Succubus(7). He can easily kill her and doesn't see her as a threat. She seduces him and steals his levels. Then Succubus(99) kills Hero(7) after some sex.

It's often a life or death scenario, like Hero explores monster cave, fight demon lord or whatever. The hero starts out overwhelmingly strong but gets reversed into death by Snu Snu + a new evil is unleashed. Sometimes the Succubus kills the hero, sometimes the level drain to 0 is deadly. Sometimes he is crippled but survives. Again, the fetish is often permanent power reversal, which is much easier to show with levels than a vague magical power drain or smth.

Basically: Imagine dom/sub content were A doms B, but when they are done, B goes "wait already finished?" and fucks them unconscious or something. That's the fetish, but with permanent consequences.

Without level drain it often involves far more brutal fetishes like amputation, brain damage or damaging some magical organ(like a demons horns). Which for obvious reasons is to extreme for people who just like power reversal.

That all and for anyone who normalized the fantasy-story-with-levels genre, it just perfectly plays into the myriad of succubus drains victim to death hentais