I joined the bounty at bountyhunters.io since 3 months ago, and I was also very surprised when asked to give a private key. and honestly as long as I followed the bounty this time I asked for a private key. and in my opinion for whatever reason asking privatkey is a very wrong act. And Bountyhunters.io, and I can't confirm and mention bountyhunters.io is a scam but the one who definitely asks for privatkey is a crime.