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What I always do is just do a wide search and click on the price column, and have it sort lowest to high price.
It might be that free don't show, as I say, I've been away. It used to.
Obvious question then. Do you KNOW there are some currently there?
https://steamdb.info/sales/?min_discount=95&min_rating=0
https://store.steampowered.com/
look on the left >free to play games click on that and scroll down
https://store.steampowered.com/genre/Free%20to%20Play/
You know he's talking about FREE games, not free to play, right?
To be honest, that was going to be my next suggestion. Try to use a third party site like that to do the searching for you and go from there.
Not a perfect solution, but if it works...
Top of Store. Games > Free to Play.
https://store.steampowered.com/genre/Free%20to%20Play/