Apple essentially killed Flash.

They decided it was a crappy technology and wouldn't support it.

What happened? Since supporting Apple devices was worth it, people stopped using Flash.

WordPress could do the same for old versions of PHP.

At Soflyy occasionally our customers encounter problems with our plugins not supporting extremely old versions of PHP.

A simple email to the customer saying "Upgrade to a supported version of PHP: http://php.net/supported-versions.php. If your web hosting provider set up your account with a version of PHP this old, they are not a reputable web hosting provider. You should switch to a modern web host before your websites are hacked." is fine. I've sent countless emails like this, and I can't recall anyone ever complaining that we don't support their insecure and ancient version of PHP.