When testing #3120731: Incorrect "Drupal already installed" if any database settings are wrong or unsatisfactory I noticed that if you have a site on MySQL 5.7 or using the mysql56 driver, and you move to MySQL 5.6 or remove the mysql56 driver, that the site operates just fine and gives no notice that you're on an unsupported version of MySQL.
This occurs when updating Drupal 8 sites or 9 sites. While this may not be a large issue now, when 9.1 starts to introduce MySQL 5.7 specific changes, sites could start blowing up randomly without any warning.
Additionally, if you install Drupal 9 say locally on MySQL 5.7 and PHP 7.3, but deploy it to a cloud with 5.6 and/or php 7.2, you may never know the requirements are not being met. Worse, you wouldn't know from the application there is even an issue. See screenshot below for what you get when you run in PHP7.2:
Proposed solution
Unsupported Databases and PHP versions should throw an error on the status page, or a critical error not allowing the site to load at all.