Does it actually matter?
I shall inform you of a secret. There are some big servers around that don’t defederate anything and aren’t defederated by anyone. :) that’s all I’m saying, it’s up to you to find the treasure.
I made a tool just for that: https://federation-checker.vercel.app/
This uses two things: https://fba.ryona.agency/?domain=lemm.ee and https://lemm.ee/instances (do replace “lemm.ee” with whichever instance you want to check)
My tool is bugged, and lemm.ee does defederate instances, so be aware of that and maybe use what my tool uses instead of my tool if you want reliable output :)