It seems that in last couple of weeks option to hide owned products from search of all discounted games has been removed.

Without this it is hard to filter those games and I ended up adding 10 games I already owned in the basket.

Can this option please be placed back?

Seconded. I'm sure there was a reason to pull "Hide owned products" as an option from the game search page, but it's a hassle for us on the user side.

It's not intentional. It was confirmed by a blue to be a bug and GOG is reportedly working on fixing it.