I assume that the LTC2000's datasheet parameter "Operating Temperature Range" is the ambient temperature. The LTC2000C has an upper limit for this at 70 °C.
The actual temperature inside the chip depends on the ambient temperature, but also on the PCB's thermal properties and on the air flow. A heat sink will of course also affect the relationship between the junction temperature and the ambient temperature.
I assume that the relevant temperature is the junction temperature (and not the ambient), so what is the corresponding junction temperature for the LTC2000 when used at 70 °C "Operating Temperature"?
The 70C is referring to the temperature of the die. Keep the die lower than 70C with a heatsink or use a higher temperature grade.
So the "Operating Temperature Range" should be interpreted as "Operating Junction Temperature Range"?
And it shouldn't be listed under "ABSOLUTE MAXIMUM RATINGS" (Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device)?
Operating conditions are not the same as Absolute maximum number. At 70C we guarantee that the part will meet its specs. If you go slightly higher than that the part won't fail catastrophically. If you exceed the Abs max specs, you are in danger of actually damaging the part.
"Operating conditions are not the same as Absolute maximum number."
-Exactly, that is what I meant when I wrote that the operating conditions shouldn't be listed under "ABSOLUTE MAXIMUM RATINGS".
You are probably right. I'll bring it up with the datasheet writers.