I don't know if that's the case or not. I have an '89 K3500 with a TBI 454. I bought this truck a few years ago and was never thrilled with its fuel economy, but otherwise was pleased with it until last November when it was totaled in an accident.
I kept the salvage for parts and recently discovered this truck has no catalytic converter. Its got a home made looking dual exhaust under it. It also has an oxygen sensor.
So my conclusion is they can function, but I don't know that it functions properly. It was not throwing any error codes that lit the check engine light.
My plan is to use that TBI 454 in my '54 GMC 1-ton, so now I'm going to need to find the same answer you are after. My project isn't far enough along adding catalytic converters is a problem, but better to know now...
