Based on my research into this topic there appears to be two reasonable solutions. Perhaps one solution is more reasonable then the other.
One key is to recognize that constraints exist as a concept outside of the instance of a class and should be outside of the actual instance data.
The first solution is to adopt SHACL. It is a W3C recommendation and ...
Speaking as a practitioner, this is my best understanding of the open-world-assumption - I never really found a good explanation of its impacts. I would kindly invite anyone to correct or extend on this post, as it would allow me to learn as well.
The open world assumption pertains to the "web" part of the Semantic Web - i.e., its open, distributed nature. ...
If you're looking for ontologies:
But there's maybe 5k ontologies, how do you hope to overview them and learn about them? You should start from a specific (narrow) problem domain.
Are these required and recommended properties only a thing which is understandable for the Google search algorithms to work well?
Yes. Follow additional guidelines in the documentation for your specific type. E.g., for Movie: required properties are name and image.
See also https://github.com/schemaorg/schemaorg/issues/1715.