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I did not change the pins numbers from the DIL package in the schematic with the metal can pins numbers in the datasheet so I cannot see how the output can go to 0V.
The current limiter is the output current in the 0.3 ohm resistor causing a voltage drop that turns on the top BD140 transistor that turns off the other transistor. When a transistor with a fixed base-emitter voltage (fixed output current in the circuit) is heated then it conducts more current. Then the top transistor conducts more which turns causes the other transistor to conduct less.
It sounds like the transistors are balancing the temperature but they do not since their load resistances are vastly different then the lower transistor has much more gain than the top one. Then when it heats the bottom transistor reduces the current limit.
The current limiter is the output current in the 0.3 ohm resistor causing a voltage drop that turns on the top BD140 transistor that turns off the other transistor. When a transistor with a fixed base-emitter voltage (fixed output current in the circuit) is heated then it conducts more current. Then the top transistor conducts more which turns causes the other transistor to conduct less.
It sounds like the transistors are balancing the temperature but they do not since their load resistances are vastly different then the lower transistor has much more gain than the top one. Then when it heats the bottom transistor reduces the current limit.