Yes, ISPConfig is compatible. But you should not change the default PHP version of the oS as other software might fail. The guide that I posted the link to install PHP 7.x as additional PHP versions so that you can use it in the website while keeping the original PHP as system PHP.
I am trying to use phpMyAdmin v5.0 and that requires the latest version of PHP, but if switch the default 7.0 PHP version to 7.3 to make phpMyAdmin v5.0 work then ISPConfig stops working. I think I will have to wait until ISPConfig and other software is fully compatible with latest version of PHP in order to be able to switch the server default PHP version. Thank you!
PMA v5.0 is still alpha and I dont think that it is safe to be used for now. You also do not need to change default php to 7.3 to use PMA with php7.3, you just need to set the vhost for PMA to use php7.3.
