It is more related to sick leave and general scarcity of teachers. I guess before the existence of timetable apps they were better in finding creative ways around those changes.
In my experience, teacher absences just meant substitutes, another teacher swapping over / handling two classes, or an impromptu class period spent watching something on tv.
I'm not sure that was much better than just telling the students in advance when you know a schedule will change and sending them to the library to study or etc.
