So I recently purchased an 8GB Pi 5 and discovered Pi-Apps[pi-apps.io] which installs Steam without any faff. (Running on the default Raspery Pi operating system). So naturally my next question was, Can it play Portal/ Half life? Ans: yes it can!
What else can it play?
From the 300 or so games in my library which have linux ports and Steam allows me to install, the following work (no tinkering was done, apart from reduction of video settings if I could):

Runs without sound:
Bastion, Brutal Legend, Galcon Fusion, Guacamelee! Gold Edition, Rocketbirds: Hardboiled Chicken, Shatter, Stacking, Superbrothers:Sword and Sworcery EP, System Shoch 2, Transistor.

The Deponia games run, with sound, but every spoken dialog line clips after a couple of words.

Runs:
7 Billion Humans, 8-Bit Commando, Amnesia: The Dark Descent, Anodyne, Aquaria, Avadon: The Black Fortress, Baba Is You, Bad Hotel, BADLAND GOTY Edition, The Bards Tale, Bridge Constructor, Broken Sword 1: Directors Cut, Broken Sword 2: Remastered, Cave Story +, Chaos on Deponia, Cogs, Counter-Strike: Source, Darwinia, DEFCON, Democracy 3, Deponia, Don't Starve, Don't Starve Together, Doodle God, Door Kickers, Driftmoon, Duet, Dungeons of Dreadmor, Dustforce, Dynamite Jack, EDGE, Edna & Harvey:Harvey's New Eyes, Eryi's Action, Eufloria HD, FEZ, Fortix 2, Frozen Synapse, Gateways, Gnomoria, Gone Home, Goodbye Deponia, Hacker Evolution, Hacker Evolution - Untold, Hacker Evolution Duality, Half Life 2, Half Life 2: Deathmatch, Half Life 2 Ep 1, Half Life 2 Ep2, Half Life 2: Lost Coast, Half Life Deathmatch: Source, Half Life: Source, HOARD, Hotline Miami, Human Resource Machine, illumine, Invisible Inc, Ironclad Tactics, Kinbgdom Rush, Left 4 Dead 2, LIMBO, Little Inferno, Little racers STREET, Monster Loves You!, Neverending Nightmares, Osmos, Papers Please, papo & Yo, Penumbra (Overture, Black Plague & Requiem), Please, Don't Touch Anything, Portal, Portal 2, Proteus, Psychonauts, Q.U.B.E: Directors Cut, Retro City Rampage DX, RUSH, SATAZIUS, Savant- Ascent, Serious Sam 3: BFE, Skulls of the Shogun, Snapshot, Solar Flux, Space Pirates and Zombies, The Stanly Parable, Steel Storm: Burning Retribution, Super Hexagon, Super Meat Boy, Symphony, Tales of Maj'Eyal, Teleglitch: Die More Edition, Terraria, Thirty flights of Loving, This War of Mine, Tiny and Big: Grandpa's Leftovers, Titan Attacks, To the Moon, Toast Time, Toki Tori, Unepic, Unmechanical, Uplink, Waking Mars, Wizorb, World of Goo, Worms Reloaded, Xenonauts, You Must Build a Boat, Zen Bound 2

Not an exhaustive list, but a good start?
What else runs on a Pi 5?