Identifying old system junk I've been using Mac a long time, &amp; none of my new Macs have ever run in a truly new state since the first Migration Assistant back with what we now call Classic Mac OS. Over time ther performance of, what should be snappy Macs, has deteriorated &amp; I put it down to accumulated junk from years of migrations. I think mostly in /Library &amp; ~/Library, but also any of /etc (/private/etc), /opt, /usr, &amp; /var (/private/var).

Can anyone point me to a quality guide to identifying junk in those locations, that safely can, or indeed should, be disposed of?
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When you use Migration Assistant, which has gotten more competent at its job over the years, outdated macOS System files are not transferred over.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.