So the question would be how to install EOL versions of PHP, I believe. Because even if DirectAdmin allows 10+ PHP versions to run at the same time, you will still have issues to install PHP 5.x and PHP 7.x on new OS#es.
And since Directadmin dropped old versions, I believe they won't complete the feature. Sorry to say that. And this is only my view, I'm not a DirectAdmin choice-maker.