Valve have launched the Zombies vs Vampires Fest on Steam running until June 2nd 5pm UTC. There's a number of nice games discounts, so here's some suggestions to go for that will work on Linux, SteamOS and Steam Deck. There's also numerous demos featured during the event too and a couple of new items in the Points Shop.
Also Valve did another event trailer:

Direct Link
As for some recommendations, I've gone through it and here's my quick picks I think will be worth your time. Specifically, all titles below are Steam Deck Verified so they will work on Desktop Linux and other SteamOS systems too.
Left 4 Dead 2 - 90% off (how do you still not own this?!)
Zombie Army Trilogy - 90% off
Death Road to Canada - 90% off
The Evil Within - 75% off / The Evil Within 2 - 80% off
Dying Light - 70% off
The Last of Us Part I - 50% off (hasn't gone lower on Steam)
Legacy of Kain Soul Reaver 1&2 Remastered - 35% off (best discount so far!)
Vampire Hunters - 35% off
See the full sale event over on Steam.
Left 4 Dead 2 - 90% off (how do you still not own this?!)Is I have no friend a valid answer ?
Insert forever alone meme
Is I have no friend a valid answer ?You don't need friends to enjoy Left 4 Dead 2.
Insert forever alone meme
I played the whole thing by myself several times, and it is a great game. There are a few parts where playing with bots does lack a certain human touch, but otherwise the bots do a pretty good job. Sometimes even better than people, because they play at your pace.
So go play it. It's worth it.

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