Quickly creating an issue for this; proper issue summary will follow.

https://www.drupal.org/docs/8/upgrade/upgrade-using-web-browser#pre-upgr...

If the database credentials to your source database are correct, the upgrade review page will appear as illustrated in the screenshot below. This page shows a summary of the upgrade status for all installed modules on the old site.

image

Latest iterations use this help text instead:

The new site is missing modules corresponding to the old site's modules. Unless they are installed prior to the upgrade, configuration and/or content needed by them will not be available on your new site. Read the checklist to help decide what to do.

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/3577
GitHub Issue #: 
3577