That's not true. I use CentOS 6.10 with PHP 7.3.13 from remi repo. Only PHP 7.4 is not possible.
PHP 7.3.13 (cli) (built: Dec 17 2019 16:10:55) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.3.13, Copyright (c) 1998-2018 Zend Technologies
with Zend OPcache v7.3.13, Copyright (c) 1999-2018, by Zend Technologies
CentOS release 6.10 (Final)