Update: Only Steam Workshop items for Counter-Strike: Global Offensive, Dota 2 and Team Fortress 2 will require approval from Valve moderators before they go live, a representative from the company has said. The new rules are designed to combat scammers promising "free skins" to anyone that subscribes to Workshop items, the representative said via email.
Users with a large number of voters or subscribers will also be exempt from moderation. Workshop items for other games are entirely unaffected.
Mod submissions to the Steam Workshop now require Valve approval before they go live. The change, spotted this weekend by a user on the CS:GO subreddit, applies to new worskhop mods and updates to existing items, and Valve says the approval process should take "less than a day".
Submissions to the Workshop will have to go through a two-step moderation process: first, new items must be verified by email to make sure they're genuine. The email will arrive at the the address associated with your Steam account within half an hour. Second, new submissions will go into a moderation queue, awaiting Valve approval.
If you're updating an existing Workshop item, subscribers will still have access to old versions, but the update will require Valve's rubber stamp. "You'll be able view and edit the content during this process, but other players will not be able to view changes until they're approved," Valve said on a Steam support page.
It didn't say whether the rules apply to all games or not. I suspect not: the sheer volume of items would surely be impossible to manage. I've reached out to Valve for clarification.
Valve didn't provide a reason for the change but, as noted on the CS:GO subreddit, the Workshop has been home to spam and scam items for a while now. For CS:GO, these items usually promised free keys in exchange for downloading, and scam accounts would often downvote legitimate maps to promote their own submissions, as outlined in this video.
It should help make the Workshop less crowded, and provided Valve is as quick as it say it will be, it shouldn't cause much hassle for creators.