Not if it’s based on known job history, personal connection, or other factors that make a candidate known to the one offering. Interviews are typically a means of getting to know a candidate. If you already know them, there’s no need for interviews. I personally know quite a few devs I’d call up to chat about a job/project and give an immediate offer to if they were interested.

Admittedly, in the context of recruiters, it could be different, but doesn’t have to be. Personal relationships are a strong signal for many people.

It isn't if you know the person hiring you and they know you.

Even then, I would expect to have a at-least a small chat for both of us to understand how each other's plans can fit in before accepting an offer.

That's a completely different situation though.

Red flag warning of what?

I've never heard of a job without an interview.

A red flag regarding the nature of the company. No interview shows lack of diligence. If there is no diligence about who they let in, what else are they not diligent about?