Developer says it's a Steam issue, but there may be something more going on.
Ready or Not screen

Ready or Not, a Rainbow Six-style tactical FPS has been delisted from Steam. The game's Steam page simply redirects to the main Steam Store index now, as is common when games are pulled from the service.

The reason for the game being delisted seems to be up in the air at the moment. A concrete reason was not provided by any party yet. Developer VOID Interactive made a tweet along with a post on Steam that acknowledged the removal of the game, but seems to express ignorance about what happened. The statement seems to suggest that it's an issue with "Steams [sic] Backend."

There is an issue somewhere with Steams Backend and it's resulted in the game being taken off the store for some reason, but Ready or Not is still on the platform.
Case closed. Just have to wait for Valve to fix whatever issue it is with Steam and... What's that? There might be another reason?

According to a report by NME, Ready or Not was pulled from Steam after it added in a level depicting a mass shooting at a nightclub. The Steam delisting came a few days after the Supporter edition of Ready or Not added a new map set in a nightclub named Prysm.

Prysm takes place in the aftermath of a terrorist attack at a nightclub, and depicts the scene of a mass shooting, with piles of bodies strewn across the map. The level bears some resemblance to the 2015 Bataclan shooting in Paris, as well as the 2016 shooting at Pulse nightclub.
Not only that, but VOID Interactive added this map to the game on June 12, the anniversary of the Pulse nightclub mass shooting where 49 people died and another 53 more were injured.

If the game was intentionally pulled from Steam by Valve, this wouldn't be the first time they did such a thing. In 2018. Valve pulled a game named Active Shooter after receiving complaints from parents of school shooting victims.

In 2021, VOID Interactive split with publisher Team17 just a day after the developer announced that Ready or Not will have a school shooting level.

Update June 18 at 1:13PM (ET): Ready or Not is now back on Steam and is able to be purchased. According to a tweet sent out by VOID Interactive, the game was taken down from Steam due to a "suggested trademark infringement" in the game's new "Night Club" map (mentioned above).

VOID Interactive says that they have removed the potentially infringing materials from their game. They have also removed the infringing references and media from all of their social channels and websites.

According to some user responses, the issue was that the name of the nightclub used in-game was very similar to the name of a real club based in the United Kingdom. You can read their full response below.

Special thanks to K-16 for mentioning that there was an update to this story.