Airdrop is a free token given by a company just to make them popular or just to introduce the project and the team were really need a KYC as proof that the token or coins was given to a particular person and I think there's no problem if it is a legitimate company , but if you don't want to share your private data ,then it is better not to join .
rules regarding data privacy have been regulated in many countries, that's just the reason ICO project owners or others require using KYC to find out who the recipients of their tokens are, in fact nowadays everyone can easily create shit tokens, they get your data just by exchange it for their shit token, is it worth it?
The problem is how can you be sure that this project or company is legit?, many of this Airdrop project appears to be real and legit, after a while or after distribution of their fake token, they will have access to some of the hunters address and one will letter be hearing stories like, I was scam or my wallet was hacked. I think personal details is a privacy that is not supposed to be given out anyhow, so let's just be careful and know who we are given our details to in order to be safe.
I also experienced the same as you, because in the past I followed a lot of devs from airdrop projects for KYC, now in my email I get a lot of spam messages and it's very annoying.