I agree - it is impossible to say "it has been thus" until an investigation report is out. This is why I wrote "unlikely", not "certainly not".
But just for discussion purposes and beyond the scope of this incident:
A feathered propeller with its engine shut down is expected to stand still - it might move momentarily, but nowhere near the mentioned 220rpm of a feathered, but still driven propeller and even way farther from the 850rpm of a thrust-producing one in cruise (numbers for DH8D). The centrifugal force induced by an off-center, unbalanced object, such as the
bird carcass that remained in the spinner is F=m ω² r. M and r is constant; at 2 rpm the figure for ω² is 0,04; at 220rpm 530 and at 850rpm 7923 - the force creating an imbalance obviously rises proportionally to this figure.
In case this is true, there must have been noticeable and certainly abnormal persistent vibration caused by this damage while the engine still was running. The "Engine Failure/Severe damage" checklist is definitely applicable in this case and will lead to an engine shutdown.
If the spinner was just dumped without any persistent vibration, it certainly is a case-by-case decision to be made on the other hand.