Various people in the Upgrade Status issue queue are concerned about the module requiring Composer though. While Drupal 8 or 9 do not require Composer to work, getting any extension installed on your site that depends on third party components is a sizeable challenge without Composer. With Drupal 8 core dependent on various third party components, it was inevitable that a dependency manager will be needed to build Drupal sites. Upgrade Status itself does require various third party components to check for deprecated API uses and even to locate your Drupal site root. So you cannot avoid using Composer to even check for Drupal 9 compatibility.
If you are not using Composer yet, the best case scenario is you use this opportunity to convert your Drupal site to a Composer based solution. If you started your Drupal 8 site from 8.8.0 or later, the file structure is already ready for Composer. For Drupal sites started on prior versions, the move is more work, as explained in the guide.
In case you are not ready to convert your site to Composer yet, but still want to get a Drupal 9 upgrade readiness check, you need to set up a separate, parallel site with Composer just for this purpose. Your existing site content or configuration does not matter for Upgrade Status checks, you only need to replicate the enabled projects on the site you use.
Install Composer first. It is best to use Composer 2 because it will be much faster and will use significantly less memory, but you should have no problem installing Upgrade Status with Composer 1 either. After that, run these commands to create a new Drupal site from Composer, add core developer dependencies and add this module as a requirement.
$ composer create-project 'drupal/recommended-project:~8.9' d9readiness
$ cd d9readiness
$ composer show drupal/core | grep versions
$ composer require --dev drupal/core-dev:[copy version above]
$ composer require drupal/upgrade_status
Then, copy all of your custom and contributed projects to the
web/profiles folders. (You could use Composer to add at least the contributed ones, but since you are not planning to use this build for anything else, this will be the quickest way). Finally, visit
/admin/reports/upgrade-status, and run the report.
This should help you get a summary of where your site is at. I would suggest you plan for converting to a composer based setup and use composer for dependency management going forward. You will only need composer on your development environment and can still use your usual workflow to push your fully built codebase from there.
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