Is there a way to downgrade a 4.1 project to 3?
Itch.io embed won't work.
Only by doing it manually.
DaveTheCoder wdym by manually?
mrnull wdym by manually?
Make a backup of the project, then try to open it in Godot 3. Fix any errors that appear. It could require rebuilding scenes, making changes to scripts, etc. There's no documented procedure for doing this.
This is an Itch-specific problem. You could try bugging guy who runs it to fix it. His twitter handle is... nevermind, he moved over to the fediverse, he still posts occasionally at https://mastodon.social/@leafo
DaveTheCoder
Okay, switching directions, how would I add these headers in a GitHub pages site?
I don't know. I'm working on my own GutHub problem. Solve that one, and I'll look into yours. 
https://github.com/orgs/community/discussions/78998
Did you read the github issues link I posted above?
DaveTheCoder I did it earlier but that was before I decided to switch to github pages. it would be easiest that way.