Thevenin voltage for a simple circuit
We ignore the R10 because effectively there is an infinite resistor across the two output terminals. Which means you have the same voltage drop at the output as is dropped across R9 which is 16volts 22*(R9/(R9+R8))

(Just think of it as another voltage divider of an infinite resistor and R10. The infinite resistor gets all the voltage across it with nothing dropped across R10.)

The memory aid of an infinite resistor across any open terminals, makes those out-lying dummy resistors (Like R10) easier to understand.