Detect a missing PWM signal in the smallest possible board space
This circuit should do what you want, within your space requirments.

At 2 kHz, pulses will arrive at 500 µS intervals.  I set the timing for the 74123 multivibrator to timeout at 2 ms. 

You wanted an active high output when the pulses disappeared, by unfortunately, in small packages like this, there is no \$\mathsf{\small \overline{\text{Q}}} \$ output, so I had to include an inverter.  Otherwise that would have come for free.



Both the 74123 and 7404 are in BGA type packages. The SN74LVC1G123, in a YZIP package is 0.9mm x 1.9 mm, or 1.71 mm².  The SN74LVC1G04, in a X2SON package, is 0.8 x 0.8 mm, 0.64 mm².  Since they are BGA, they can be placed as close together as the pick and place machine will allow.

The two caps and resistor all come in 0201 packages, which are each 0.6 mm x 0.3 mm, say 0.9 mm x .35 mm footprint on the PCB.   So the total for all three is 0.945 mm².  Thus the total area for all five components (excluding room for traces) is approximately 3.3 mm².