[jhgb]

I'm not sure how this changes anything. Is your PII in the order forms/sales contracts mentioned above? If yes, you'll have to delete those as well anyway, right? If it's not, the order forms/sales contracts themselves don't have to be linked to something that may potentially get deleted.

Please note that by "immutable" I don't mean that data won't get deleted eventually, just that it won't be deleted until nothing needs it anymore (and until then won't be mutated either), so basically the same thing that languages like Haskell mean by "immutable". Then, once you don't need it (= it's not observable anymore), it could perhaps get archived or erased, whatever you prefer.

[inopinatus]

Then it isn’t master data anymore; it’s just one field of a record of a commercial document. This is taking a long way around to the same point.

(Presumably no-one is trying to reference-count GC their RDBMS. If so, I wish them all the luck in the world.)

[jhgb]

I'm not quite sure what "master data" means in English (a non-native language to me) but Wikipedia tells me that it's "data about the business entities that provide context for business transactions" (and lists examples that sound relevant for this situation to me). Based on that I'm inclined to think that this would qualify.