Is it safe to have 3-switch light-vent-heater on 20 amp circuit with another light switch and a GFCI outlet?
I am sure someone else can cite code, but generally speaking if the hardwired loads on a circuit are &gt; 50% of the capacity of the circuit then you can't have ordinary receptacles on the same circuit. With a 20A circuit, that means if you have 10A of fixed load (1300 W on a 20A 120V circuit qualifies, plus you have some other lights as well).
The reason for this rule is that if you used a hair dryer (bathroom example) or toaster (kitchen example) or similar item that draws 1500 W and the heater was on at the same time, you would trip the circuit breaker very quickly. Nobody likes nuisance trips - this rule helps avoid them.
Bottom line: A heating fixture pulling 1300 W should be on a circuit by itself (which includes the light and fan that go with it) and possibly with other lighting, but should not be on the same circuit as any general use receptacles.
As far as why the heater was disconnected previously, that could be:

Nuisance trips!
Malfunction - easier to disconnect than to replace.