I found some resources online indicating that ORO has/had VAT ID / Number property on the Customer. Searching through the codebase I couldn't find any traces of VAT ID / Number. Is this still a case in the latest 4.* version? Also, does the Customer has properties such Company Registration Number?
As last one, what would be the best approach of extending the Customer entity and adding this fields?
The vat ID field is available in the german version of OroCommerce application: https://github.com/oroinc/orocommerce-application-de.
For existing OroCommerce instances you can install this bundle https://github.com/oroinc/german-localization that provides an integration with wirecard, infinitepay and dpd.
The vat Id field is available there, but it is implemented as a part of infinitepay integration https://github.com/oroinc/OroInfinitePayBundle. You can install it separately as well.
If you are not using infinitepay, to add a vat id field, you can copy the code from there to your customization,