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It’s rotted out due to too much make up water. Most likely leaks in the system.

We’ll find out once the heating cranking over next few weeks and we’ll go from there

Thanks all.

The filter is a demineralize filter which goes in on everyone of my boiler replacements.

Also possibly some sediment ended up in the runnout and is creating a dam that is holding water. The internal parts of the valve coming apart also could form a one way valve that lets steam in but does not let water out. If the valve is installed up side down it could potentially not let condensate drain. What does it look like compared to the other valves?

I assume you checked that the valve is all the way open.

Condensate doesn't have to block the pipe to stop the radiator from heating and it usually doesn't block the pipe. You just need a pool of condensate sitting in the pipe. When the steam hits the liquid water it condenses in to water until it has more or less evaporated the water. Steam won't progress beyond the water until it has heated the water to steam temps

Those are Trane in wall convectors or a copy of them.

@EdTheHeaterMan I bought a 24" pipe wrench at HF today. If I can break it free, I'll order the #2 Gorton.