Which BJT-parameters need to be derated at raised ambient temperature?
Take a look at this explanation of Absolute maximum ratings as defined by the IEC. 

source: http://www.electronics-cooling.com/2004/02/the-temperature-ratings-of-electronic-parts/


  The International Electrotechnical Commission (IEC) [5] defines absolute maximum ratings as “limiting values of operating and environmental conditions applicable to any electronic device of a specific type as defined by its published data, which should not be exceeded under the worst possible conditions. These values are chosen by the device manufacturer to provide acceptable serviceability of the device, taking no responsibility for equipment variations, and the effects of changes in operating conditions due to variations in the characteristics of the device under consideration and all other electronic devices in the equipment. The equipment manufacturer should design so that, initially and throughout life, no absolute-maximum value for the intended service is exceeded with any device under the worst probable operating conditions with respect to supply voltage variation, equipment component variation, equipment control adjustment, load variations, signal variation, environmental conditions, and variation in characteristics of the device under consideration and of all other electronic devices in the equipment.” In other words, the part manufacturers select the AMR values, and the companies, which integrate electronic parts into products and systems, are responsible for assuring that the AMR conditions are not exceeded.


so based on this standard the it is up to the part manufacturer to specify their absolute maximum ratings such that they work "under the worst probable operating conditions". Of course whether or not a specific manufacturer actually does so is another story.

Again as was mentioned by others absolute maximums are the point where the part can be assumed to break so it is best design practice to stay well away from that limit under all conditions.