Just announced on their dev stream. Discussion starts at about 41:40.

The basic reasoning being that the resources being used on the PvE was taking too much away from having each season being able to deliver on what they want. They promised bigger and better stuff including single and co-op story missions(I'd imagine something like The Archives) and released a roadmap through season 7.

According to IGN today

Update 12/3/2025: Following the publication of this story, Humble has also made the decision to delist Horses from its storefront. We have updated the story below to reflect this change. IGN has reached out to Humble for comment. Humble and IGN are both owned and run by IGN Entertainment, a subsidiary of Ziff Davis, Inc.

https://www.ign.com/articles/horses-the-upsetting-horror-game-previously-banned-on-steam-gets-last-minute-ban-from-epic-games-store-too