LDO temperature expectation
For 24V in and 5V out you could have used a 7805, no need for a LDO. Or even better, a switching regulator, which would dissipate a lot less/

This LDO dissipates (24-5)*0.150 = 2.85W which is a lot for a SMD package. If you put enough vias, the heat will travel down the vias into the ground plane, which will act as a heat sink, but if the board is horizontal, airflow isn't as good as with a vertical heatsink with fins.

80°C on the tab is too much, the board must be quite hot, the lifetime of any nearby electrolytic capacitors will be shortened significantly. According to RthJC of 4°C/W junction temperature is around 91°C which is also a bit hot for comfort. 

There is really no margin to add any extra loads. Again, for 24V to 5V, use a switching regulator instead, maybe a readymade one from Traco or Murata, there are many options.

Anyway, back to your question: the dropout curve. The LDO's pass element is a PMOS. The RdsON of this PMOS increases with temperature. The minimum dropout voltage occurs when the PMOS is fully on, thus dropout voltage is RdsON*I, and since RdsON increases with temperature, it follows that the minimum dropout voltage increases too.

This is not a concern here, as the dropout is 19V.


  I don't know how much mA the board takes, not sure how to measure this on a PCB..


Well you should have calculated this BEFORE using a LDO that will burn if it uses more than 200mA... again, not much margin for error here!