In a week or three, anyway. Anywhere somebody has bothered to liquify the NG, the radon will have decayed before it gets vaporized again. Where pipelines lead straight to gas wells, the dwell time in the pipeline before it is burnt could be quite short. But there might be negligible radon even at the wellhead, as it would need to be picked up from surrounding rock on the way out, unlike e.g. helium which accumulates.

Probably other toxins are more important: mercury vapor and hydrogen sulfide are usually mentioned. Hydrocarbons other than methane may burn to carbon monoxide or soot. Gas is said to get much less processing before delivery than it used to.

SpaceX will probably purify theirs before pumping it aboard, just to increase the useful life of their Raptor engines.

good points