TYPO3 Core

Just experimented a bit with the PackageManager and got something working, though of course not tested thoroughly, and it may or maybe not be a viable solution:

\TYPO3\CMS\Core\Package\PackageManager::getActivePackages

   public function getActivePackages()
    {
        if (empty($this->activePackages)) {
            if (!empty($this->packageStatesConfiguration['packages'])) {
                foreach ($this->packageStatesConfiguration['packages'] as $packageKey => $packageConfig) {
                    $addAsActivePackage = true;
                    if($GLOBALS['TYPO3_CONF_VARS']['SYS']['enableExtensionByFeatureToggle'][$packageKey] ?? false) {
                        $addAsActivePackage = GeneralUtility::makeInstance(Features::class)->isFeatureEnabled($GLOBALS['TYPO3_CONF_VARS']['SYS']['enableExtensionByFeatureToggle'][$packageKey]);
                    }
                    if($addAsActivePackage) $this->activePackages[$packageKey] = $this->getPackage($packageKey);
                }
            }
        }
        return $this->activePackages;
    }

So I my additionalConfiguration.php file, I would of course have my feature toggle settings and this new one:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['enableExtensionByFeatureToggle']['news'] = 'myFeatureName';

As some points out on Slack. My suggestion to a solution might bring big trouble as patching getActivePackages for runtime extensions might introduce a lot of DI-cache and other TYPO3-cache issues...

For my example, one suggests me make anything in the extension B gated from the feature toggle as well. That could be a lot of conditions all over. Another point is, that this might be a 3rd party extension, and not a in house extension. So that would require to hack the extension instead.