sysupgrade -h will provide help text, which includes
-l | --list-backup
list the files that would be backed up when calling
sysupgrade -b. Does not create a backup file.
The list comes from metadata about the installed packages' config files to start. It can be modified to include additional files/directories by editing
For example, on one of my boxes, to handle things that I configured (not part of packages):
## This file contains files and directories that should
## be preserved during an upgrade.