Also note that the OP said “avoid extending classes”, but didn’t say “avoid implementing interfaces”, so they don’t disallow inheritance in a wide sense of the word.

I think “avoid extending classes” is there because it is as good as impossible to design classes that can be extended easily in ways you do not foresee, and if you do foresee how your classes could be extended, it often is easier for your users if you made your classes more flexible, to start with.