To be clear:
Your Yubikey doesn’t store identifiable usernames and does not store any of your passwords. Anyone who finds your YubiKey would have absolutely no way of knowing which accounts it can log in to.
This changes a little if the person who **finds** it knows the owner, everybody else who finds a YubiKey on the street, train, cafe, airport and on and on won’t be able to figure out whose key it is.
So don’t write your name our your handle on it.