Do not copy other author's workshop items without permission, either here on Steam, mod.io, or anywhere else. It's their work, and they can ask for it to be removed.

This post mirrors a lot of what is stated in the first section of the Valve's Developer Community Wiki. The second section is not addressed here, but it is beneficial to be aware of it in any case.

At first glance, don't assume malice intent on either party. A friendly discussion can resolve the issue in many cases.

How can I use a workshop item, such as a mod, with custom changes on my server?

1. Ask the author! Many workshop creators would appreciate feedback to better improve their creation. If your changes are something others might want as well, give the author a chance to add those changes to their own item. If the author does not respond, then permission is not given if the workshop page doesn't state otherwise.
   
2. If the author doesn't want to integrate your suggestions, then ask them if it's ok to re-upload with your own changes. Many authors are also permissive on that, as long as you ask first. Follow any additional steps they provide.
   
3. Upload yours privately: Set it to "Unlisted" in Steam. Then you can use it, the players on your server can download it, but no one can see it on the Workshop.
   Keep in mind that if the author didn't approve this, they can request a takedown as well if they are made aware of it. The goal here is to avoid problems, not circumvent the system. It's likely if you ask for permission, they will request you upload it "Unlisted." This goes back to #2.

This includes so called "mod packs."

My favorite mod is dead, and the author doesn't respond!
Unfortunately that does happen. However that content is still theirs. Just because something is abandoned does not mean you have free permission to copy it. Remember, the creation is still theirs, you only have permission to use it unless stated otherwise.

Reposting a mod - With Permission

Mods may be re-uploaded or reposted with the permission of the original Mod Author. These re-posted mods should follow some basic guidelines.

Any mod that does not follow the provided guidelines is subject to moderator action.

The mod should have a link to the original.
The mod should offer proper attribution.

So as an example, a properly re-posted mod would include:

You can find the original version of this mod here <Link to the mod>
This mod was created by "<Name of the author>" and uploaded with permission.

Also, it's a good idea to add the original author as a contributor to your upload. This does not give them permission to edit the mod contents directly (though they could edit the description), but they will get a notification that they were added. This proves to the community your reupload was authorized, and will reduce the number of false reports. Some authors may not want to be added, or won't care; however offer it anyway.

What can I do if I see one of my items uploaded without my permission?
It's possible the user wasn't aware of the above information, and just wanted something for their own sandbox. Your first interaction with them should reflect that.

1. Inform them that you didn't give them permission to re-upload it, and ask them to take it down.
   
2. If you are willing, inform them you would gladly integrate any suggestions/changes they made if they make sense.
   
3. If they refuse to take the item down, report the item or file a DMCA on Steam if applicable.
   
4. Send a message to a moderator either on Steam, or Discord[discord.gg] (such as myself), with a link to the offending item, and the original. Please only message if you are the original author. Do not submit on behalf of someone else. We will investigate and remove the offending item as necessary.

What can I do to better inform users?
Be sure to include a disclaimer on each workshop page informing of the permissions you grant, or don't, such as:

This workshop item cannot be redistributed without explicit permission.
This item is not authorized for posting on Steam, except under the Steam accounts named <author(s) name(s)>.

This isn't required, but does speed up the DMCA process on Valve's end.

This is a recommendation from Valve themselves. This recommendation can be found on the DMCA report page.

Edit history:
2022-08-25: Added paragraph on workshop contributor. Cleaned up formatting.