PHP7.3 is still quite new and there's obviously plenty of life left yet in PHP 7.2 >
https://secure.php.net/supported-versions.php but we could run PHP7.3 now, if we knew for definite that XC5 had been tested and officially supports it
We're guessing that this may only be from XC 5.4.*.* onwards in Q1 next year, but worth asking now, just in case...