AFAIK a Steam Deck is a full Linux PC, so you can do anything on it that you can do on a Linux PC, including that. Whether this will be especially convenient is a different question.

• get a dock
• attach keyboard and mouse on the dock and connect the dock to a monitor
• go into desktop mode on your Steam Deck
• open up Discover and install applications

Basically a full fledged PC. There are some limitations, but for the most part you are able to code on it. You can write text and source code files, edit videos, edit images, browse the web with regular Firefox and so on. I can’t say if the Steam Deck works well creating games with Godot, but technically it shouldn’t stop you from trying.