You guys are incredibly responsive. I might try to sign a transaction with the key that allowed me to donate 5 BTC to Bitcointalk. But that goes way back. It doesn't seem simple to me!
It's easy with Bitcoin Core bitcoin-cli: Just call createwallet and make sure you tell it to create a legacy wallet, that will allow you to sign messages, then use importprivkey, and finally use signmessage.
You might also see if you still have access to your old Ripple addresses; signing a message with one of those would work equally well. Seems like you used this one a few times:
The question is, is it possible to sign a message from a ripple address, just like it is with bitcoin and Ethereum? I have done a quick search and there isn't much information, plus things look too complicated rGd4FaNjg22EvTuot3SRKF1suueUSc8Lhd
One thing that makes me wonder is who could just forget about their donator account and just continue using a lower ranked account for all that long without appealing.
You can, apparently.