I am not sure that is something that applies 100%, but I understand the concern.
It is my understanding that we should try to build solutions to current problems, and be open to future use cases that could involve small additions in functionality.
It would be stupid to design an unmodifiable system just because some parts can be deleted and we are not sure what future needs are. Code should always be easy to extend, in my opinion.
Conversations like this are always difficult to discuss at a high level because the way we implement the words we use can be very different. Code can be written in a way that a lot of complexity is added in order to make it extensible, or it can be written in a way where simplification is used to make it extensible. Both authors would agree that extensible is good.
It is my understanding that we should try to build solutions to current problems, and be open to future use cases that could involve small additions in functionality.
It would be stupid to design an unmodifiable system just because some parts can be deleted and we are not sure what future needs are. Code should always be easy to extend, in my opinion.