You might encounter this deprecation message after your upgrade to symfony 5.1 in your symfony project:
Since symfony/dotenv 5.1: Passing a boolean to the constructor of “Symfony\Component\Dotenv\Dotenv” is deprecated, use “Dotenv::usePutenv()
To fix this, go to config/bootstrap.php and remove the
false from the Dotenv constructor:
Actually the file config/bootstrap.php has been removed from symfony 5.1 and was replaced in public/index.php.
But since the project was updated from an symfony 4 version the bootrap file is still present.