Can someone explain how to do the sliding speed glitch? I can't seem to find a tutorial anywhere and I haven't been able to replicate it...

This is a category suggestion: get a villager to master level. I have found that the easiest way this is to use a mason like so:

Collect 60 clay and 76 stone. 60 clay ---> 6 emeralds = Apprentice^ 4 emeralds ---> 4 chiselled stone^ 76 stone ---> 4 emeralds = Journeyman Collect 60 diorite/granite/andesite^^ 60 diorite/granite/andesite ---> 4 emeralds = Expert 10 emeralds ---> 10 terracotta = Master ^ For these trades, you need to click them individually to get the stone discount. ^^ The mason will have 2 journeyman trades, and there is a 5/7 chance that at least 1 of them will be a stone variant.

After not too long I got a sub 10, but this category can certainly be pushed much further with better smelting efficiency, mechanics and village proximity.

I've submitted a run in the Kill Warden SS category well over a month ago and it still hasn't been verified. Is this just because it is a category that nobody cares about, or are they just too busy verifying more important runs?

Is it allowed to use a resource pack that changes the normal splash texts to speedrunning splash texts?