From a box directadmin when installed with custombuild 2 and PHP in CGi-modes allows to switch PHP version per domain bases at user level. Currently there can be used only 2 PHP versions.
Why:
Simple if supporting more versions you have to support / ADMIN also all other internal and external, third party software for working 100% in all these PHP versions. ( optimal settings/configs and a lot more of that sh...t)
Something breaking then is a real risk.!
If professional Productive Webservices you don't take such risk and extra Support/Admin time, with the LOW prices for extra Servers. (all versions on one Server you already need then more resources so why not tuning a extra Server optimal for that other Version? )