The documentation at https://www.mongodb.com/docs/mongoid/master/reference/fields/#custom-field-types should be extended to cover the changes made in Mongoid 8.

Specifically:

Whether mongoize, demongoize and evolve should be written to accept any values (yes, this is already mentioned).
That evolve should pass through unconvertable values.
That mongoize and demongoize should return nil for unconvertable values.
That Mongoid will always call demongoize with values that were retrieved from the database (and hence are of a limited set of types) but applications may in theory call demongoize directly with arbitrary input.