My Steam library currently sits at an embarrassing total of 322 games owned. I buy and redeem codes for a lot of games on Steam. But I do sometimes buy them on other stores or other platforms entirely. Undertale, for instance, I decided was a perfect game to play on my Nintendo Switch. So when Steam's Discovery Queue recommends it to me, I debate what to do. I treat Steam's machine learning systems like a new puppy: constantly at risk of picking up bad habits if I tell it the wrong thing.
So I just skip it every time I see it, afraid that if I hit "Ignore," Steam will think I don't like Undertale and will show me fewer things like it. I do like Undertale, I just don't want to buy it again to prove a point to Steam. I also don't want to clog up my wishlist with things I actually own. Fortunately, Steam has now added a separate checkbox in a dropdown under the "Ignore" button called "played on another platform."
Valve's Alden Kroll says on Twitter that "the plan is to feed the new 'played on other platforms' data into new recommendation engines," most specifically the new Interactive Recommender tool. This new feature is a way to signal to Steam that a game is of interest to you even though you don't want to buy or wishlist it.
If you've used the "Ignore" function for Steam games in the past and would like to change how you'd tagged them, you can see a list of your own ignored products here, though Kroll says you shouldn't feel compelled to do so.