These kinds of things are exactly why foreign keys are so helpful to keep things from getting corrupted. Invoices should always reference a user in the user table.
The relationship should not be "ON DELETE CASCADE" but probably "ON DELETE NO ACTION". Then when some junior dev tries to issue a DELETE on the users table, the DB will correctly tell them to get bent and throw an error because there are dangling invoices still associated with the user. Because you are a smart person, you've also prohibited DELETE and UPDATE's against the invoices table for the db user that is used. The application cannot delete the invoice, and it cannot delete the user either--which is the only correct course of action. The database can now protect itself from hostile input from all your junior devs. Exactly why you want FK's. You can't trust user input.
The fact that the author of the article shudders about cascading deletes demonstrates that they have zero authority to speak about databases--something that is very common in the developer community. DB's aren't mysterious black boxes. They are really fucking cool tools that can do all kinds of neat shit to keep your data nice, safe, cozy and warm (they do way more than that too... a good database is perhaps one of the most underutilized tools developers have in their toolbox)
The relationship should not be "ON DELETE CASCADE" but probably "ON DELETE NO ACTION". Then when some junior dev tries to issue a DELETE on the users table, the DB will correctly tell them to get bent and throw an error because there are dangling invoices still associated with the user. Because you are a smart person, you've also prohibited DELETE and UPDATE's against the invoices table for the db user that is used. The application cannot delete the invoice, and it cannot delete the user either--which is the only correct course of action. The database can now protect itself from hostile input from all your junior devs. Exactly why you want FK's. You can't trust user input.
The fact that the author of the article shudders about cascading deletes demonstrates that they have zero authority to speak about databases--something that is very common in the developer community. DB's aren't mysterious black boxes. They are really fucking cool tools that can do all kinds of neat shit to keep your data nice, safe, cozy and warm (they do way more than that too... a good database is perhaps one of the most underutilized tools developers have in their toolbox)