The main reason, at least to me, is that the routing won't really evolve over time to be better and better, because without glitches, there's pretty much only the intended route.

It would be somewhat like LMOE, and LMOE was a terrible category.

but the current wr is glitchless Kappa, but actually this game is incredibly boring glitchless, there’s not much interest into a glitchless category other than you, it would be very annoying to define what qualifies as a glitch and what does not as this game has some very unclear lines between a glitch and not a glitch. And it would just kill co-op making co-op glitchless essentially pointless

expert realism glitchless could potentially be interesting, but other than that, meh idk

As mentioned, it is impossible to define clearly enough what is or isn't a glitch. Would we allow bhops or not? Tank boosts? Idle warping? The last two are absolutely intended behavior that are just abused. It would also make the runs very straightforward and uninteresting since it's just running from point A to B while trying not to get hit or specialed which is already what happens. Co-op would be Solo plus three other people suiciding each map, and hardly anything additional would be played in the maps in Solo since the really big skips are all Co-op.

I see glitchless more as a solution for games which have absolutely massive skips or sequence breaking that bypass large portions of a game entirely, meaning that the only way to run through all of the game's content is to have a category that disallows skipping it. That isn't the case in L4D where we still experience almost all of the content, only some parts are skipped in Co-op with GL boosts.

Basically, LMOE but on newest. And expert realism if you wanted it to be any slower.

But really if you want to decide what's allowed and what isn't I think you would have to do it by categorising them and not individually limiting each glitch otherwise it becomes too arbitrary, e.g. movement instead of bhops

As Master said, there's nothing in L4D2 that is really game changing demands disallowing because it changes the game entirely. It's already not actually that different than playing normally.