@nicksweb
No, that's saying you cannot upgrade to newer versions if you are using osTicket pre-1.6. v1.6 is extremely old so you can ignore that statement.
Anyways, it's literally so simple. You download v1.16.1, unzip it, make full backups of your current site files and database, upgrade to PHP 8.0, copy over all the v1.16.1 files from upload/ to your running site, login to osTicket, and run the upgrader. If anything goes wrong (which it shouldn't) then restore everything from backup and retry.
Cheers.