Given that there is only one day left, you will highly likely not be on Drupal 9 tomorrow. So what happens to your Drupal 8 site once the core software is end of life? As I wrote two days ago, unless you are on Drupal 8.9.x you are already running on end of life software. As prior versions of Drupal 8 don’t stop running, Drupal 8.9.x will also not stop running tomorrow. There is no expiring license key that will stop the site from functioning. There will not be a banner at the bottom of the page that the site is insecure. In fact the site will not even be insecure immediately. However, there will not be security fixes to Drupal 8 anymore. So the next time a fix comes out for Drupal 9 that may be applicable to Drupal 8, that fix will not be made anymore to Drupal 8. Depending on the nature of that security problem, you site may be in no trouble or big trouble, but the distinction will be left to you to decide.
Using Upgrade Status and Drupal Rector automated code fixes, the upgrade from Drupal 8 to 9 is still the easiest in the last decade (assuming you are already on Drupal 8.9), so I would highly suggest to plan to do the upgrade soon and don’t risk staying on Drupal 8 for too long.
There are also various changes to drupal.org projects and issues. These will likely not happen immediately tomorrow, but will be done soon. For contributed project maintainers on Drupal.org, releases that are only compatible with Drupal 8 will be marked unsupported as well, much like the same process that happened to Drupal 6 last time. Testing setups that are against Drupal 8 will be removed. Issues submitted against Drupal 8 will automatically be moved to Drupal 9.2.x (where bugfixes are still possible). If they are not applicable to Drupal 9 anymore, the issues will later be closed by people.
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