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From my experience:
The max collector current is limited by the 100mW max dissipation of this device and only 6mw of dissipation is realized at 5V Vdd. Higher irradiance would decrease Vsat but with a collector load resistance in the 1 to 10K of ordinary operation, 100mW at 5V would not be realized and Vsat would further be further reduced by increased irradiation, lowering dissipation.
IIMBG, depending on the distance of the radiating source from the phototransistor, the irradiating energy heating effect with the phototransistor touching the photo-emitter device would not be sufficient to raise the phototranistors temperature beyond the safe operating temperature. You could measure the max heating effect yourself with a thermistor or other temperature measuring device. The phototranstor's leads themselves would function as a heat spreader into the PCB or breadboard to dissipate any heating. The irradiance itself (IMHO) would likely not damage the phototransistor, apart from any heating effect.