Identifying old system junk

Touch nothing. Bigger problems come from files and software from 3rd party developers who have no (or limited) uninstallation mechanisms, which then leave cruft all over the HD, and which may have installed files in locations other than what Apple recommends. System files and apps are updated, modified and erased with successive macOS updates.

When you use Migration Assistant, which has gotten more competent at its job over the years, outdated macOS System files are not transferred over. I've used MA with Time Machine backup disks for moving my user accounts to four new iMacs starting from the days of macOS Leopard.

At my job, we use MA exclusively to move our users to new laptops when the replacement cycle comes up. Users are given an eternal Time Machine drive which a laptop and are instructed in its use. IT staff have no time, and our users don't have the expertise, to move data and apps manually when replacement time arrives. We also require our users to use the cloud-based file storage (Google Drive in our case) for their work-related files.