MacBook Air virus warning notice

I understand your point. If you start seeing problems with Safari after an update, I bet the first thing you would do is shut those Extensions off just to verify it is not a software conflict. You have helped many people solve problems they are having and I'm sure some were even Extension conflicts. Running extensions with Safari requires a user to be extra diligent to maintain their system and with your experience I know you are, but some others may not be.

In general: Any reading material or pop-ups referencing “virus” or “hacker” is a scam or an advertisement (or too often, both), until proven otherwise. Not a virus. Not a hacker.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

If these are sliding in the from the top right of the screen, a site you visited has given itself permission to send push notifications. Everything you're seeing is nothing but a scam.

Open Safari's preferences. Click on the Websites tab and then scroll down to Notifications. Clear any entries in the right hand window. You might find something similar in the Pop-up Windows heading below that. Block any you don't recognize.

Then if you want, uncheck the box below as shown here.

I personally can't think of any reason why I would ever want any website to push notices to me.

Not an attempt to break into your computer, just a Notification from a website that should not have been allowed. The solution provided by Kurt Lang will solve your problem.

Also make sure you have not allowed undesirable Pop Up Windows at the same location. While you are at that setting there, select Extensions and review those as well. It is best not to have any extensions installed.

It is best not to have any extensions installed.

A good idea, if you truly have no need for them. But I have two installed for Safari. One I can't do without (1 Password), and the other that makes using the web in general far less annoying (Ka-Block!), an ad blocker.

You are also correct on your point. Shutting off all installed extensions is a standard trouble-shooting step when you're having difficulty connecting to sites, or Safari is acting badly in general. A really poorly written extension can even affect the system.