I see on solscan.io account pages that most of them have isOnCurve false, but my delegated staked account has isOnCurve true.
Does this mean someone has the private key of this account?
Who?
By "most accounts", I'm assuming that you mean associated token accounts, which are program-derived addresses from the associated-token-account program.
Stake accounts, on the other hand, are rarely program-derived addresses, so clients generate a new throwaway keypair for every stake account.
Without knowing anything more about your stake account, there was definitely a keypair for it at some point, since you needed to generate a signature when creating the account. I can't tell you who still has the keypair though. Thankfully, since the stake account is owned by the stake program, the keypair is effectively useless. It can only be reused if you delete your stake account.