--snip--
Here is a msg I signed with it back in 2016:
-----BEGIN BITCOIN SIGNED MESSAGE-----
mill0601 from bitcointalk today is 6 nov 2016
-----BEGIN SIGNATURE-----
<15d4cdy9TgUyLstnVf7MDDaPK5tM6UepKH>
<IHRWO++bWTwcMfF2Bpq1eNTERIE4Saq1Ty059BQZD5ePJOfyhlRguSUdvyLuHQylKG2J/C/4iagzCiCwxMb6IMM=>
-----END BITCOIN SIGNED MESSAGE-----
now don't say that I have copied it from somewhere. I have copied it from my account's inbox. I've signed it back in 2016. but never posted it in public.
--snip--
How would this message proof anything? Signing a message is providing proof that you are in still control of the same wallet you used to generate an address that was staked a long time ago. You can't proof that you are the same person who staked an address by providing a message that has a message timestamped 4 years ago...
If a signed message is asked, this means that you have to create a new message with the current timestamp to proof without a shadow of doubt you still controll the same wallet. In this case, this message only proofs you have access to the inbox of this account