I’m assuming you’re importing into a blank WordPress site, and not an established WordPress site. If so, then you should be free to experiment with an outdated plugin… as you shouldn’t have any fears of the plugin breaking your site.
Install and run the plugin and see if it works. Ignore any “Warning”, “Notice” or “Deprecation” errors.
If it doesn’t work, downgrade your WordPress to an older version, perhaps 4.9 (5.0 brought a lot of changes) and try again. If this works, you can later remove the plugin (after you’re done using it) and update the WordPress installation to the latest version.
Also, please note the very first sentence in the plugin’s description: “Imports well-formed static HTML files into WordPress” (emphasis mine).
If the code on the pages is a jumbled mess of in-valid HTML code (as if I wrote it :-)), the plugin is going to have a hard time working correctly for you. For instance, even though your site displays the address https://www.RowleyResearch.org, it seems all the pages live on a different address https://rowley.one-name.net/ — and are merely embedded into the former address using the ancient frameset tag.
Even with “well-formed” HTML code, you’ll still need to do some tweaking in the plugin’s settings to control what to import and what to leave out. For instance, you surely don’t want to import the navigation menu — as that will leave you with a static navigation menu on every page that cannot be managed centrally using the tools provided by WordPress.
Finally, no matter how perfect your HTML code may be and no matter what tweaks you do, you’ll most likely still have to open each page and clean up a few things manually after the automated import. With this in mind, you may as well just copy and paste manually ?? .
Good luck!
