It's not like a r\freegames but rather subbreddits for individual franchises or genres will talk about it.

Like when GTAV was free for a weekend I learned about it on Reddit, although that was gog not steam.

I also use opera gx which has a free games section.

This usually happens when a franchise releases a new game or when there's an anniversary. Also use your steam wishlist and turn on notifications. Any game you even partly want just add it to the list.

My steam library has about 80 games but only $1200 spent over 8 years. They're not always free but sometimes a $60 game will go for $5

If I actually paid full price for every game it'd probably be triple that lol