Do mods not work on Official Dedicated Servers?

[Sexy gay shenanigans]

I have about a dozen mods installed, all are weapon skins, and one pack of skins for the Survivors. About one in five servers I join actually allow me to use my mods. Do Official Dedicated Servers disable add-ons? And if so, what was the point of implimenting the Workshop if add-ons are disabled by default?

[Gimmeh Jibbs!]

Some/most Official dedicated servers are pure, which means they will enforce vanilla gameplay. Not all of them are, though, and you can't really choose.

[Sexy gay shenanigans]

Is there any way to tell which one's do or don't?

Again, just saying, what the hell was the point of implimenting the workshop if it isn't even fully compatable.

[Gimmeh Jibbs!]

As far as I know, there's no way of telling in the selection process. The only way you can tell before fully loading in is either via custom loading screens or other indicators (if the server is pure it will revert the loading screen back to default), or by checking the developer console. If the developer console says anything about "sv_pure 1" below the "connected to ISPwhatever", then it's pure and will enforce vanilla.

Valve seems to have reinstated some of their official servers, most of which seem to allow mods, but if you want a greater chance of un-pure servers try connecting to a Best Available Dedicated. Most fo them allow mods, but some of them have very weird other mods going, like moon jumping and other gameplay-affecting mods. Again, it's a bit of a tossup which ones you'll get.