Afox I'm afraid it is just another OS... and PHP 7.4.x is still rapidly approaching EOL, regardless of who adds fixes to it. While the PHP 8 repo may not be covered 'officially' by the Debian security team, it is maintained by one of them (see https://tracker.debian.org/pkg/php7.4 and look for Ondřej Surý signing the PHP packages you're currently using!), and the actual PHP development team!
Again, you'll find a lot of actively maintained systems stop supporting EOL packages, not just osTicket - that's life, especially given that cyber security is now a primary concern for most organisations. Basically, your options are now:
- keep using osTicket 1.15.x until Debian 12 comes out, expected in January 2023, and then upgrade
- use the PHP 8 repo linked in my previous message
- import the fixes from 1.16.x into 1.15.x yourself (unless you're a developer, this may not be feasible)
- move to a different distro (something tells me you really don't want this option 😇 )
First option may be easier, and if it were me I'd use the second, but it's obviously all up to you.