As mentioned, the symbol in the drawing is for a standard diode. A zener diode would work, if the breakdown voltage was greater than the solenoid drive voltage. A zener would limit the supply voltage to the solenoid while also provide flyback voltage path when the solenoid shuts off.
If they were not installed, either they were not installed and should have been or the drawing shows more details than strictly necessary and the diodes are built into the solenoids (which is very possible, many solenoids and relays have the diodes built in).