Classes/Console/Command/UpgradeCommandController.php
public function checkExtensionCompatibilityCommand($extensionKey, $configOnly = false)
    {
        $this->output(\json_encode($this->upgradeHandling->isCompatible($extensionKey, $configOnly)));
    }
public function checkExtensionCompatibilityCommand($extensionKey, $configOnly = false)
    {
        $this->output(\json_encode($this->upgradeHandling->isCompatible($extensionKey, $configOnly)));
    }
Checks for broken extensions

This command in meant to be executed as sub process as it is is subject to cause fatal errors
when extensions have broken (incompatible) code

@param string $extensionKey Extension key for extension to check
@param bool $configOnly
@internal