Unity Asset Bundle 9 Jan 2019 
The chinese version of Unity has its own inbuild option to encrypt AssetBundles/BundleFiles. As it's a feature of Unity itself, and not a game specific protection, it is included in UnityPy as well.To enable encryption simply use UnityPy.set_assetbundle_decrypt_key(key), with key being the value that the game that loads the budles passes to AssetBundle.SetAssetBundleDecryptKey.
Unity Asset Bundle 9 Jan 2019
The server checks both individual asset integrity and Unity assetbundle integrity to prevent cheaters from modifying their files to gain an advantage. Previously it was possible to bypass this by loading a different assetbundle on a per-asset basis, for example by adding a legacy ".unity3d" assetbundle to a vanilla vehicle. Individual asset integrity checks now include the assetbundle hash as well which prevents this exploit as long as the multi-platform ".hash" file is available for the server.
Unturned is currently using Unity 2019 LTS. Upgrading to 2020 LTS seems to be stable now and fully backwards compatible with existing content. If you would like to give it a try it is on the "unity-2020" beta branch, though there are still some known issues to fix next week. Assuming there are no unexpected surprises *fingers crossed* it should be in the next update.
Modders can optionally include ownership metadata in their asset bundles now. If the game is loading an asset bundle from a Steam workshop file but the file ID does not match then loading will be cancelled. The intention is to help prevent asset bundles from being easily copied/stolen.
Terrain materials, along with a few miscellaneous other file types, were still being kept in an older asset bundle format. Unlike the newer format these files did not have integrity tests in multiplayer, so players could delete the terrain materials for an advantage. All vanilla usage of the old format has been replaced, as well as on the Hawaii and Greece maps. Support for the old format will be kept for the foreseeable future, but please consider upgrading.
The game has been updated from Unity 2018 LTS to 2019 LTS on the "unity-2019" Steam beta branch. There are still a few bugs to sort out, but it takes advantage of several new performance optimizations if you are curious to test it.
For content creators:Good news: there's a new tool to make bulk-exporting the .unity3d asset bundles much faster. I've updated all of the vanilla content to use this new system so I had to go through it as well, and Vilespring was kind enough to update all of the France bundles.
Master Bundles allow content (textures, meshes, audio, etc) for multiple game assets to be bundled at the same time, rather than into individual .unity3d files. (This is probably how it should have been in the first place, but anyway...) While slower to build, they have the benefit of updating many bundles at once, and are more RAM-efficient because any common files between the bundles are shared rather than duplicated.
When Unturned finds a MasterBundle.dat file it will assume all child folders are using this bundle with paths relative to the .dat file, unless Exclude_From_Master_Bundle is specified in the individual asset .dat file. For an example of this, check out the Greece/Bundles and Hawaii/Bundles folders.
Another effect of this change is that it will be easier to separate some maps from the game to free up space for more future maps. Specifically, I'm planning to move Hawaii + Germany and then Greece + Cyprus, to the workshop rather than in the main download in a future update. All Germany assets will continue to be bundled with the game, only the map itself will be moved. Hawaii and Greece assets will be available as a separate download on the workshop for maps which would like to add them as a dependency.
For the moment only Unturned's theme song is available officially, but adding custom music is relatively straightforward. Export audioclips from Unity into an assetbundle ending with .content. Then create a stereo song asset from the asset browser (right click) and drag/drop the audioclip into the song slot. The theme song example file can be found in Bundles/Assets/Songs.
Unity 5 is now entirely open and supports the asset bundles Unturned uses for custom content out of the box. If you're interested in creating your own workshop mods you can check out Unity 5 personal edition.
The annual Unity Mega Bundle is on right now, a collection of Unity assets bundle together with savings up to 95% off. The Bundle is organized into tiers, where higher dollar value tiers include all the tiers below it. The Mega Bundle tiers include: