Unlike a device with positive temperature coefficient, the NTC thermistors (negative temperature coefficient) can be used by themselves as active devices that provide a negative resistance, which can be used to make amplifiers and oscillators, exactly like with any other diodes with negative resistance, e.g. tunnel diodes, IMPATT diodes, Gunn diodes, Shockley diodes, diacs and so on.
Nevertheless, I do not think that anyone has ever made amplifiers or oscillators with thermistors, because unlike the diodes where the negative resistance has electrical causes, the inertia of the heat transfer in thermistors makes the achievable upper limit for the amplified frequencies very low, typically under 1 Hz.
A device with positive temperature coefficient could be used as an amplifier or as a switch (like a relay) only together with a separate heater, as you say.
I have seen old organs which used solid state VCOs that also had an incandescent lightbulb near the circuit boards to help maintain a stable temperature, and had thought they must use a thermistor although I seem to be mistaken as I can't find much information about that.
I did find this however:
https://northcoastsynthesis.com/news/temperature-compensatio...