You pay nothing to receive airdrops (free coins) , however you share data . so only share relevant info to such airdrops , don't share kyc detail , use secondary email , never share private key , never share complete details about you . If you follow such steps then you will lose only 2-5 minutes of time but not the money .

There is a Telegram Channel that announces airdrop scams. https://t.me/airdropscams

A lot of good points made about airdrop scams in this thread. I have an airdrop telegram network and one of the resources I use to test an airdrop before posting is to check if the airdrop and website url has malware. I use this tool for that: https://app.webinspector.com/public/tasks

What is the purpose of a scam airdrop?  I thought they were supposed to be free

Those things above should really be on mind that if we do talk about airdrops it should really be free but we do see some projects do ask to sent out ETH before you would able to claim those free tokens which its really dumb for you to send anything just to get out those tokens. If someone do ask then its a 100% scam and they are just taking advantage for those noobs who havent much experience or knowledge into things and carelessly done these things and later on they do realize that they had been scammed already.

4% airdrop is real, 96% is scam. very project need KYC by messenger Mew is scam,

I agree. Even some airdrop listings are scam! For example at www.airdrops.io all the rating figures are cooked! There is supposed to be a honest rating system where everybody can vote (+ or -). A project can go from -100 points to 500 in less than an hour and the opposite is true also. There must be corruption there. Some CEOs buying the ratings. I don't trust those now. I personally make my own due diligence and do not check those rating list scams.