Sometimes when you go through Steam's endless library, you'll stumble across games you've played elsewhere. The Interactive Recommender has added a new alternative to "ignoring" games while still showing your interests.
Via PC Gamer:
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.
Using the "ignore" function can signal to Steam that you don't like or are simply not interested in a game. Whereas this new feature will show that you're interested, but you've played it elsewhere. This allows for the Interactive Recommender to be much less likely to mess up your personalized queue.