Can someone explain the purpose of R53 in this old linear power supply circuit
R53 shares the load current with the pass transistor, probably to allow a lower-rated transistor and/or smaller heatsink than would otherwise be required.

The Zener diode D9 will prevent the output voltage from rising too high if there is no load.

I'm guessing that this is a dedicated supply within some equipment.  If so, knowing the expected loads, the designer may do things that wouldn't be done in a more general-purpose supply.