A new Steam feature will now allow users to mark games they own on other platforms so that Steam will stop recommending it to you and so Steam can better tailor your recommendations using games you don’t own on Steam.
The new feature will show up on a game’s store page. There’s a new drop down arrow beside the usual “Ignore” button that now allows you to select “Played on another platform.”

Hopefully, this tip will help to make your Steam page just a bit tidier.