I like to call that style "modeling a taxonomy of the world". Even real-world carefully studied taxonomies change all the time, good luck adapting your code base when an employee is also a customer. It was a crap style from the very beginning.

Smalltalk style OOP has its own issues, but there's lots of good aspects to it and you really have to experience Smalltalk as a complete system to really get it, it's nothing like Simula/C++/Java-style OOP.

That said, best thing you can learn in your programming career is to get rid of "Customer" classes (or whatever the equivalent is for your particular problem). A customer is a unique entity represented by an ID, a private key if you will, which links data in various systems.

Need to split some part of your codebase that handles authentication into a separate microservice for whatever reason? No problem, the same key can be used to refer to data in that system.

Your program magically becomes more modular, more maintainable, easier to understand, and more performant, all in one go.

Use OOP when you need an abstract interface to something that can be implemented in various different ways, e.g. data structures or plugin systems, stuff like that. Any time I see "PODs" full of getters and setters I cry on the inside (and sometimes on the outside).