So these new features are backwards compatible in that old code that has no typed properties will continue to work. This is a minor release and PHP tends to be

So these new features are backwards compatible in that old code that has no typed properties will continue to work. This is a minor release and PHP tends to be almost completely BC even between major release versions so it's unlikely you'll face any issues. If you want to start writing code that uses these features in an existing library then you can branch a new version that depends on PHP 7.4, and define that as the requirement in composer. That way users with older PHP versions will pull older versions of your library.