My 68 has it. The 68 wiring diagram shows the four terminal relay and the key-in terminal in the ignition switch, going to the drivers door switch. 67 wiring diagram shows only a three terminal relay, and I can't see a key-in terminal in the switch. Doesn't mean the later 67 didn't have them.

The wiring diagrams in the 68 service manual has the Pontiac and Tempest relay named 'Horn relay and buzzer alarm', they have the Firebird relay named 'horn relay'.

In the removal and replacement section of the 68 Chassis electrical service, they call it the horn relay and buzzer alarm.

Page 10 of mt 1968 pontiac owners manual states"all models are equipped with an anti theft key warning system".

So I agree, we can safety assume The 68 Firebird have the buzzer alarm horn relay.