Edit:
I'm able to sign a message with my old address which had been assigned to my old account back then when there was a feature to include your bitcoin wallet address inside your profile page (or is it for member rank and up only? idk i forget xd).
What do you mean "include your bitcoin wallet address inside your profile page " ?
I believe that you don't understand the sign message. When you sign a message, you need to have a private key of that address/ wallet. The point to prove your account ownership is here: Sign a message from an address that was used and quoted before the hack. It is to prove that today, you are the owner, not the hacker, and still have control on that address and private key.
It is easy to include any bitcoin address to your personal text, and website link and others in the Profile page. I can search those addresses if I know the username of your hacked account and include it into those places. I can do it without control of private key for that address and it is non sense.
What i meant is that i still have the access to my old wallet address that i mainly use at 2014 - 2016, which yes, i still do have the private key, but unfortunately in the past i've never ever did a signed message using that address. And yes I'm still new to this signed message thing, and i got a question, does signing a message with said address need the private key?
I'm sorry for my misunderstanding about how signed message work tho, i'll read more about it.