as a time served turner i am wondering why you would want to use a sharp root on a taper thread where there is potential to damage both in storage and application of the tap and plug and in the tool grinding and chaser maintenance on a pressure vessel.

Dave

I hesitate to respond, as there are far more knowledgeable folks here than me, BUT my assumption has always been that the softer plug metal would be deformed by the boiler material and the interference fit would lock the plug in place so it won't vibrate loose, and provide a more reliable steam tight fit. As I see it, the sharp bronze edge is deformed by the steel to fill any gaps in the thread form.
Hope this is correct!!

UK washout plugs are of several teeth per inch and various tapers but all work by an interference fit of the taper