Unless you count Clipper as a "backdoor", this article asks the same question I am. The whole point of Clipper, of course, was that keys were escrowed.
Clipper was deliberately backdoored (the key exchange had a trap door), with that backdoor only publicly found after its release. This was more the a just key escrow. Why would that not count?
The entire point of Clipper was to field cryptography that NSA could break. That wasn't a later revelation. It was the understanding at the time. It's why there were "the crypto wars".