Twitter could, of course, have just given a warning. But I also don't see any problem treating the tweet as if it were current. The age of the tweet does not change whether it is acceptable or not, does it?
Well, he was banned for violating the Twitter rules, and those rules didn't exist at the time.
Getting a message saying something like "this message violates the rules today, so we're going to delete it" or "delete this message within a week or we'll have to ban your account" would seem more reasonable to me than immediate ex post facto enforcement.
That’s interesting question. Following this principle, should we ban historical books that suggest to e.g. kill self-claimed witches, treat nonbelievers as second-class people and so on?
This isn’t really a punch in the face though, is it? If someone discovered you used racial slurs nine years ago, I think they would be justified in choosing to no longer associate with you or ask you to recant what you had said in the past.
Getting a message saying something like "this message violates the rules today, so we're going to delete it" or "delete this message within a week or we'll have to ban your account" would seem more reasonable to me than immediate ex post facto enforcement.