Was the Convair 990 unique among airliners in its ability to reverse engines in flight (resulting in the ability to perform very steep approaches)?

You ask about a very interesting aircraft, however as others have already answered the ability reverse thrust wasn’t unique.

However the engines did have a unique feature.

Convair 990 Coronado — Wikipedia

The engines were also changed to the uprated General Electric CJ-805–23s, which were unique in that they used a fan stage at the…