This page is our investigation of crossplay game The Mound: Omen of Cthulhu. We always verify information manually using official sources - no AI.
Yes, The Mound: Omen of Cthulhu will support cross-play at release. Players on PlayStation 5, Xbox Series X|S, and PC (via Steam and Xbox Store) will be able to play together.
We emailed the development studio directly to confirm cross-play support at release and received a response confirming it will be available, as shown in the screenshot below.

You can disable cross-play in Mound: Omen of Cthulhu by going into the game menu -> Settings -> Options -> Gameplay Options -> Crossplay -> No.
We CAN NOT recommend doing this, as you may lose the ability to find parties more quickly or even at all.

Unfortunately, at the time of writing this investigation, there is no easy way to send cross-invites to a party, but we have described a workaround below.
For the same platform, like Steam to Steam, PlayStation to PlayStation, and Xbox to Xbox, work fine cause game uses native platform invite features.
As cross-platform invites are not possible, you can use the workaround. To play The Mound: Omen of Cthulhu together with friends on different platforms, you need to host a public game, so your friends can connect to it using server browsing

Worth to mention that developers have already mentioned that they are working on cross-platform invites to make them easy [source]