Starting in 12.1.0, link_file is not functional. To upgrade to version 12.1.0, any reference to link_file must be removed. However, simply commenting these out can result in 500 errors when attempting to use the relationship.
This healthcheck document has some information about the upgrade failure reason:
It states: "When you create a new link_class value, do not forget to update the autoload class map: composer dump-autoload"
Minimally experienced at custom development, I don't comprehend how to apply this line or what follows in that document.
Would someone be able to spell out for me how I am expected to develop a relationship so that it includes auto loader as expected and does not error out?