I have no idea. I thought it was because of invisible viewmodels as someone explained to me. But I am sure Valve banned it for a good reason so you will have to ask them instead of us.
It is because of invisible viewmodels. These exploits only applied to competitive servers because viewmodels are forced in competitive. Valve removed them because these directories were being exploited to remove viewmodels in competitive.
They can't be exploited in casual play because the command is actually allowed on casual servers. The specific exploit can be found
here.
Casual and Competitive servers share the same sv_pure whitelist. I guess it's easier for valve to remove it on all their servers instead of just applying these settings to competitive or maybe their doing it for the sack of consistency.
A well known modder called Paysus (who made most of the viewmodel animation mods) has made a detailed post about the issue
here. The post has numerous reasons to explain why invisible viewmodels don't give any advantage to the player.
This whole situation was mainly because valve wants to force viewmodels in competitive. People found a way around it so they blocked the directories.
I just want to make sure everyone understands why this happened and hopefully convince you guys that viewmodels are safe to allow. With that said, I'm just trying to provide the information I'm currently aware of. Me and many other people have emailed valve about the issue but they seem to be very strict about forcing viewmodels in competitive.
Sorry to continuously bother you about this. I don't play on valve servers often (mostly due to sv_pure) and there aren't too many community servers with good playerbase like skial. So that's why I'm personally really bothered by this change.