"Collection" would indicate you are using Lightroom-Classic (or v6 & earlier). Answer for Classic-
A collage you create in the Print Module will only be 'saved' if you click on the tab to [Create Saved Print]

This creates a 'Print' Collection (in the Collection panel) that uses the Print Template AND the selected images, so that the 'Print' can be recalled in the future should you want a reprint.
Note: a Print Collection uses the small Printer icon.
A Print Collection is 'dynamic' ie. you can add photos to this Collection, and you can [double-click] to re-enter the Print module with the saved print.
Saved Collections only exist in a Catalog, so if you created a new Catalog (when you upgraded) then- No, your Print Collections will not be in the new catalog.
Is there anywhere where adobe specifically states what gets backed up in the lrcat database ?
A Backup of the Catalog IS the .LRCAT database! Including Collections (BUT NOT the physical photo files.)
If you have 'lost' the Print Collections then you may be able to recover them from a previous Backup of the Catalog. See this link for instructions when you would target (select) the Print Collections to restore: RESTORE PARTIAL from backup catalog?
You can also [Print to a JPG] as a way of preserving a Print job.