How to Edit Your Steam Profile’s Privacy Settings

• With Steam open, navigate to your username on the top of the screen, and select Profile from the drop-down menu.
• Click the Edit Profile button that’s located beneath your Steam Level number.
• You should see several descending tabs with profile information on the screen’s left side. Click Privacy Settings at the bottom of the list.
• Mouse over the Game Details and select Private.

source: https://www.pcmag.com/how-to/how-to-hide-the-smut-that-youre-playing-on-steam