Constant Safari security popups

You need to go to Safari and click the Safari menu, then choose Settings > Websites > Notifications. Remove all websites you see there. Those Notifications you see are because a website asked if they can send you Notifications and "Allow" was chosen. It is that website that took your permission to allow the Notification to let them send you these that are nothing more than an Advertisement attempting to scare you into downloading some software to solve a problem you do not have. The website is paid for that advertisement just like any other form of advertisement.

In any case, removing all websites you see in the Notifications section will solve the problem and you will not see anymore of those unless you allow a website to send them to you.

I would recommend to leave it unchecked. This will stop most websites from asking about notifications, but there are some sites that do not respect that setting and the permission request will still come up, but you just need to make sure to choose not to allow the notification. In either case, you will not get the Notification unless you Allow them at that prompt.

Please see the very informative user tip below. 

Stop unwanted Notifications - Apple Community