Games like Down n' Up
Games like Down n' Up include Celeste, Super Meat Boy, VVVVVV, SANABI, and Jump King. We matched them by the tags players give Down n' Up on Steam (Precision Platformer, Action, Difficult, Platformer, and 2D): closest matches first, and among equally close ones the better-known games.

Celeste
Shared: Precision Platformer · Difficult · Pixel Graphics

Super Meat Boy
Shared: Precision Platformer · Difficult · Platformer

VVVVVV
Shared: Precision Platformer · Platformer · Pixel Graphics

SANABI
Shared: Action · Pixel Graphics · 2D

Jump King
Shared: Precision Platformer · Difficult · Pixel Graphics

Geometry Dash
Shared: Difficult · Precision Platformer · 2D

Katana ZERO
Shared: Action · Pixel Graphics · Platformer

Downwell
Shared: Precision Platformer · Pixel Graphics · Platformer

Skul: The Hero Slayer
Shared: Action · Indie · Pixel Graphics

Shovel Knight: Treasure Trove
Shared: Platformer · Pixel Graphics · Action

Hollow Knight
Shared: Platformer · Difficult · 2D

Pizza Tower
Shared: 2D Platformer · Platformer · Indie

The End Is Nigh
Shared: Precision Platformer · Platformer · Difficult

The Messenger
Shared: Platformer · Pixel Graphics · 2D

Hollow Knight: Silksong
Shared: Difficult · Indie · 2D

Ambidextro
Shared: Precision Platformer · Action · Pixel Graphics

Rain World
Shared: Difficult · Platformer · Pixel Graphics

Cuphead
Shared: Difficult · Platformer · 2D

Electronic Super Joy
Shared: Platformer · Difficult · Action

Hotline Miami
Shared: Action · Pixel Graphics · Indie

Gravity Circuit
Shared: Action · Platformer · 2D

Broforce
Shared: Action · Pixel Graphics · 2D

Super Meat Boy Forever
Shared: Action · Difficult · Precision Platformer

Out There Somewhere
Shared: Platformer · Pixel Graphics · Indie