Since when are threads getting deleted in this forum?

Your thread was a duplicate of an already existing thread, and it got merged. Learn how to check your post history.

Your thread was not deleted, it was merged with a different thread due to being, more or less, on the subject of the coming 1.0 release.

https://steamcommunity.com/app/251570/discussions/0/4360122995638836654/?ctp=11#c4360123163643030806

Right hand side of the forum, "Your Forum Account-> Post History", You can view all comments you have made in the past here, and you can check whether your post was simply Moved, or if it was Deleted.