Lately im trying to register in airdrops but never get any money from that.
So, i don't think Airdrop is a good idea anymore, maybe if youre luck enough you gonna get the big one such as Arbitrum and etc.
But i prefer to doing bounty works then airdrop.
You should clearly understand the differences between airdrop, which is issued for certain actions, such as subscription, retweet, and retroactive airdrop, in which the reward is given for early use of the network. And that's what the retroactive airdrop from Arbitrium was about, which you write about and registration was not required there.