One of the first things I do when reviewing whether an application can be upgraded is to look for anything that can be removed.
If it has modules that aren’t used, uninstall and remove them.
A common one for Drupal projects is modules left from previous migrations, such as from Field Collections to Paragraphs that are no longer needed but still present.
Remove any old themes that are no longer used.
If you have content types, fields or views that aren’t used, remove them, assuming you don’t need the data.
Remove unused routes, Controllers or other classes not used in custom modules, or any other unused custom code.
Less moving parts means less to upgrade now and maintain in the future.