What connection does MS Office have to Netscape Navigator?

I think you have discovered some forgotten historical artifact of Microsoft Office.

I believe that these registry keys belonged to Microsoft Office FrontPage, a discontinued WYSIWYG HTML editor and website administration tool from Microsoft, which used Netscape software.

FrontPage was replaced in 2006 by Microsoft Expression Web and SharePoint Designer, later also discontinued, but apparently Microsoft Office continues to faithfully install it to this very day. I can testify that these keys are still present in Office 2019. They are to be found in all manual uninstall instructions for Office.

If this worries you, feel free to signal it as a bug via the Windows Feedback Hub.

Note:

I have found more traces of Netscape, this time in ASP.NET. I found on my computer the files C:\Windows\Microsoft.NET\Framework\v2.0.50727\CONFIG\Browsers\netscape.browser and C:\Windows\Microsoft.NET\Framework64\v2.0.50727\CONFIG\Browsers\netscape.browser. A ".browser" file is used by ASP.NET to detect the installed browser type and define its capabilities for displaying Web page content. This means that ASP.NET, to this day, is still checking if the Netscape browser is being used (!).

This tells Netscape, if it were installed, that Office files should be opened with Office, so it avoids a dialog when you download an Office file.

The only application that would ever read key this would be Netscape, but Office installs it regardless of whether Netscape is installed, because if you install Office first, then Netscape second, the registration would be missing and the user would have to run the "Repair" option of the office install.