This is correct. It only impacts pages with page URLS.
Yes, this was a improvement added in 9.8. As pages were not used by navigation/pages/microflows they were not checked by studio pro. If you disable this setting, for backwards compatibility, pages are not checked.
As projects used to have this option, this setting was added for backwards compatibility.
Derek, have you received answers on the questions you asked?
I would like to know what the "Strict page URL checking" option in Security does too.
Info from documentation does not help a lot:
Strict Page URL Checking
When Strict page URL checking is enabled, pages that are only accessible through a page URL will also produce consistency errors for security problems. By default, this is enabled in new apps.
Release notes 9.8:
We added the option to enable checking security for pages only accessible by page URL. This is enabled by default for new apps and will also become the default in the next major Mendix version. (Ticket 129205)