Do slotted screws indicate old fixtures? How old?
I'd say by around 1990 most fixtures and devices (outlets, switches) had been shipped with combination screws (both #2 Phillips and slotted drivers work, or sometimes #2 square and slotted). The transition probably started in the mid-70s and varied by region of manufacture, backstock quantities, etc.
Answer informed by vague accumulated memory, not empirical data.