When I started using Galaxy about a week ago, I could see my Steam friends in the Friends sidebar. Now they do not appear, I only see GOG friends and Xbox friends. I have tried deleting and resetting the Steam plugin but no luck. I see nothing in integration settings about friends from other platforms.

Why was this working 4 days ago and now has mysteriously vanished?

Same, it worked a few months ago, then vanished and never worked again. I have no idea why.

because Steam changed their system to block it

Within the past week?