What is your oldest bitcoin transaction?
Probably something from 2016, when I received 10k sats from 99bitcoins.com, and lost it to some scammy web wallet.
Would you sign something to prove it?
No, because I just generated some address from a web wallet, received it on-chain, and never saw it again. Maybe on my old, encrypted hard drive, there are some screenshots of it, but it is BIOS-encrypted, and I can no longer open it, to even see the address anymore. I can only scan the blockchain, and guess that one of these 10k outputs are mine, at that time I could locate it exactly, now I don't know, where it is.
Also, there were times, when
SIGAINT e-mails were working. Today, they don't, so I can no longer prove, that I received anything.
but would not feel comfortable posting that address since it's still in use today
I still have some addresses from 2019, when I bought some BTCs from ATMs. But I don't use these old addresses anymore, because they were generated by these ATMs. But I still have the private keys to that. Again: signing it would be unwise, because it could prove, what I bought or sold, and when, because of some address reuse.
Anyway, stories like that are fun to read, but dangerous to share. And some of them can be easily faked, for example by finding brainwallets, and signing messages from addresses, which are known to be broken, to pretend, that someone is an early adopter.