Games like Slay.one
Games like Slay.one include Broforce, Terraria, Police Stories, The Escapists 2, and Duck Game. We matched them by the tags players give Slay.one on Steam (Action, Pixel Graphics, Multiplayer, 2D, and Strategy): closest matches first, and among equally close ones the better-known games.

Broforce
Shared: Action · Pixel Graphics · 2D

Terraria
Shared: Multiplayer · Pixel Graphics · 2D

Police Stories
Shared: Action · Strategy · Top-Down Shooter

The Escapists 2
Shared: Multiplayer · Pixel Graphics · Strategy

Duck Game
Shared: Multiplayer · Pixel Graphics · Action

ShellShock Live
Shared: Multiplayer · Action · 2D

The Escapists
Shared: Pixel Graphics · Strategy · 2D

King Arthur's Gold
Shared: Multiplayer · Pixel Graphics · Action

Your Only Move Is HUSTLE
Shared: Action · Pixel Graphics · Strategy

Tooth and Tail
Shared: Pixel Graphics · Strategy · Action

Timberman
Shared: Pixel Graphics · Action · Multiplayer

Superfighters Deluxe
Shared: Action · Pixel Graphics · Multiplayer

Teenage Mutant Ninja Turtles: Shredder's Revenge
Shared: Pixel Graphics · Multiplayer · 2D

Awesomenauts
Shared: Multiplayer · Action · 2D

Worms W.M.D
Shared: Action · Multiplayer · Strategy

Forts
Shared: Multiplayer · Strategy · 2D

UFO 50
Shared: Pixel Graphics · Action · 2D

Pixel Gun 3D: PC Edition
Shared: Pixel Graphics · Multiplayer · Action

Bopl Battle
Shared: Multiplayer · Action · Strategy

Stardew Valley
Shared: Pixel Graphics · Multiplayer · 2D

BOKURA
Shared: Pixel Graphics · Action · 2D

Katana ZERO
Shared: Action · Pixel Graphics · 2D

SMITE®
Shared: Action · Multiplayer · Strategy

Skul: The Hero Slayer
Shared: Action · Pixel Graphics · 2D