If they are using hot water, instead of steam, they are most likely ''charged'' (meaning they have water in them all the time).
They probably originally used steam with the gravity drain back format that was common before switching to the lower temperature hot water charged format from a boiler.
Depending on how they are plumbed, they could be shut off and drained. The cast iron radiators themselves can take a lot of pressure (even from ice) as they usually have a significant amount of air space for the expansion. But the connecting lines with the smaller diameter are at risk.
If they are PEX, then it wouldn't be as much an issue, except for maybe the brass/composite fittings.
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