Vermintide 2 has had mod support since early June, and has since then managed to fill out its Steam Workshop with a whole bunch mods ranging from quality-of-life fixes to massive gameplay changes. However, while the mods were available and fully playable, you could only actually use them on the modded server. In other words, you couldn't get any loot or cosmetics by playing with mods enabled, and you couldn't really play with random people due to various restrictions. Not an ideal situation, I'm sure you'll agree.
Thankfully, Fatshark has finally begun to whitelist mods, and as such we have now received the very first batch of mods that will fully function on the live servers! Since this is essentially a test for the whole system all of the mods are either quality-of-life changes or cosmetic upgrades, but I would say that they are still a noticeable improvement over the basic game and something I would highly recommend you check out by heading over to the official website.
Depending on how well this whole experiment goes, Fatshark will be moving on to progressively tougher cases, eventually even approving mods that significantly alter the gameplay or add new items. While I would personally love it if they just threw caution to the wind and enabled pretty much everything, I think Fatshark is rightly worried about some unseen loophole or exploit getting smuggled in with one of the mods. And as you might expect, the bigger mods have significantly more code to sift through, hence why they're being held back until the very end.
While we can't impact what sort of mods actually make it through to the live servers, there is one thing we as the players can do: vote for the mods we like. Fatshark has stated that they will first go through the popular mods in order to find potential candidates, so if you have a mod you particularly enjoy and would love to see it brought into the live servers, make sure to head on over to the Steam Workshop and toss them an upvote. It's still no guarantee they'll get in, but it does at least increase their chances!
Once more information comes in I'll make sure to let you know, but until then you should check out the current batch of sanctioned mods over here.