Over 700 years. A tower originally built in 1310. The island it’s on is an exclave that is technically part of Hamburg.

600k seems a bit too much. That’s a fairly large city.

Obviously it depends (for work when I can work on the train I am okay with longer journeys) but there has to be some sort of non-linearity involved, I agree!

Blockchains have the property of being append-only, so a blockchain is precisely what makes it impossible to delete transactions. That being said, in a distributed system, once the message leaves trusted servers, it is obviously also impossible to delete it.