after using the VPN Super Unlimited Proxy, some unknown sites sent me message randomly after 27 minutes through calendar apps?

Yes, using scam apps will create far more issues than it fixes. Delete the useless app, and verify that you have not signed up for subscriptions to the useless app.

 If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

Remove spam from Calendar on your iPhone

you'd need to ask the developers of the VPN. Hopefully their VPN service is actually what they tell you it is and they are not breaking the honor code of the internet.

and if you jailbreak an iOS device all bets are off.