Afox While that may be ideal for people running legacy systems that require PHP 7.x, the fact is it still goes officially EOL in 2 weeks, and osTicket is not a legacy system. Most software will not support EOL versions of third-party packages, even if one specific distro is maintaining them separately. Much in the same way that since osTicket is open source, you and anyone else are welcome to take the fixes from later versions and integrate them into the 1.15.x code if you wish, but that still doesn't make the 1.15.x branch current.
Also, osTicket has always only supported the two most recent versions. Even if PHP 7.x was current, only 1.16.x and 1.17.x of osTicket are maintained. I myself had to skip the 1.16.x branch while I was getting ready to upgrade (in my case, it was the fact that the php-imap extension was deprecated in Fedora), but fortunately for me 1.17.x has allowed an upgrade of my system without breaking anything.
If you need to get PHP 8 on Debian 11, have a look at places like https://wiki.crowncloud.net/?How_to_install_PHP_8_on_Debian_11 - note that I'm not a Debian user, so can't vouch for the effectiveness of this page!