Is Minecraft Speedrunning allowed on a non-vanilla server with no world-related modifications? By non-vanilla server, I mean server JARs like Spigot, Bukkit, etc. If I create a server, and it does not have any world-related modifications but only plugins, is it allowed to speedrun on it? In this case, I think using Multiverse-Core is allowed unless code modified.

please read this thread https://www.speedrun.com/speedrunning/thread/6tgel

why is it always minecraft runners who are lost? yeah its like the most popular game on the site but id still expect some other game's runners to be lost too?

There are a few of them, but relatively to Minecraft they are much less noticable.