What is the minimum supply voltage for the MC34063E in step-up (boost) configuration?
If you wish to obtain datasheet wisdom by adumbration you could look at other hints as well :-)

Note fig 12 where they could easily have taken Vin under 5V and have chosen not to do so.

5V is used as reference test Vcc in many cases.

Note that at any sort of current the internal darlington saturation eats a significant amount of your lunch at &lt; 3V Vcc.

That's an ST data sheet but I think it looks like a redraw of the very old Motorola version. What do other manufacturer's data sheets say?

In practice I have found that MC34063xxx curl up their toes at slightly under 3V Vcc.
I've run them from 3V on up with good results.