How to find the actual measurement range of a temperature sensor, HTS221?
Well spotted - the datasheet accuracy specification does not cover the whole operating temperature range.


  Does the operating temperature (-40 to 120°C) mean that this sensor will not melt off during high temperatures like 100°C, but I cannot expect it to measure that temperature?


Since it is claimed to operate over the -40 to +120°C range, then you should get a reading, if you (properly) request one from the sensor via I2C/SPI. Also, the device should not be damaged by operation within that temperature range. That is pretty much the definition - it will operate within its specification in that range.


  Does that mean that the temperature sensor can measure only from 0-60°C with the above accuracy


Yes, that is the only range where you can expect that stated accuracy (or else you can legitimately complain to the manufacturer).


  and the rest of the measurements are going to be way off?


Not necessarily - there is simply no specification outside of that 0 to +60°C range. It will likely be less accurate, but you don't know by how much, so it might, or might not, be what you call "way off". Any reading is possible, and you have no way to claim on the manufacturer if you don't like whatever that turns out to be, since they would not be breaking any stated specification with any response.