What is intelligent tracking prevention debug mode?

Intelligent Tracking Prevention is on by default. You can read an in-depth description here: Safari Privacy Overview. As far as "debug mode" is concerned, it is designed for WebKit developers, and it should be disabled after debugging a webpage. Read https://webkit.org/blog/8387/itp-debug-mode-in-safari-technology-preview-62/ for an explanation.

Does it help stop spam if enabled?

No.

Yes. Its "debug mode" is strictly for debugging purposes. It should be disabled unless actively debugging a web page, because leaving it selected will cause the page to load more slowly than it would otherwise — especially if the webpage is coded to upload tracking information to whoever wants it. Many (perhaps most) webpages want to track your every move, so that they can sell that information to anyone willing to pay for it. ITP sticks its thumb in their eye.

How can I make the Intelligent Tracking Prevention Debug Mode remain on?

I see that you say it's should be on by default.

I wrote:

Intelligent Tracking Prevention is on by default.

... so your question is?

Please review Safari Privacy Overview. There are no configuration options for ITP.