Drupal VM 5.1.0 was just released (release name Recognizer), and the main feature is PHP 7.4 support; you can now begin running and testing your Drupal sites under PHP 7.4 to check for any incompatibilities.
PHP 7.4 includes some new features like typed properties, arrow functions, and opcache preloading which could help with certain types of code or site deployments (I'm interested to see if opcache preloading could help the startup time of Drupal inside container environments like Kubernetes!).
And with the release of PHP 7.4, PHP 7.1 support was dropped—thus Drupal VM 5.1 is the first version to completely drop support for PHP 5.6, 7.0, and 7.1. In the past, it was possible (though not recommended or supported) to install these older PHP versions. As of Drupal VM 5.1 it is no longer possible. So if you need to still migrate some code from an ancient codebase running on PHP 5.6 or some other unsupported version of PHP, stick with Drupal VM 5.0 until that's done.