Update: Valve has confirmed that it now requires developers to get in touch before any release date changes can happen on Steam. Developers can still hit the release button whenever they want once the game build has been approved by Valve, but to put a different release date on their store page they'll need to get permission first.
Developers will also be sent reminders two weeks ahead of their release date to check they're on track. These emails had previously been sent out to only a small number of developers, but as of yesterday it's been expanded store-wide.
Original story: In an attempt to get a coveted spot in Steam's Popular Upcoming Releases list, some developers have allegedly gamed the system, changing the release date in the backend, even if the store page shows a different date. This way, games can appear in the list much earlier than they should, pushing other games down. Valve employees confirmed the issue in March, and it looks like the solution is now in place.
When attempting to change the release date, a warning now appears that asks the developer to contact Valve if they want to make changes. This extra check should put off developers looking to exploit Steam, though it also means there's an additional hurdle stopping developers from being able to curate their own store page.
"If you need to make changes to this date, please contact Valve here with the reason for your new release date and what date you like it set as," the message reads. "You should be pretty certain that your new date is the date you will release."
Valve's yet to confirm the change, but I've reached out for more details.