Only Up! is no longer being sold on Steam across all regions. It was not just pulled from sale; the page was removed from Valve's store altogether.
Recently, Only Up! has been enjoying surging popularity; streamers and various content creators took notice of the climbing simulator. The community gradually began to grow.
[UPDATE] Only Up! has returned on sale on Steam. It is not yet entirely clear what actually happened.
There is speculation that Only Up! was taken down due to copyright issues. Previously, certain assets had already been removed from the game. On the Steam forums, users are criticizing the developers for ties to NFTs (artwork on walls, etc.).