I suspect there is more to this than there seems at first glance. PIR foam without any fire retardant additives burns moderately though not as fast as PU foam which unmodified burns like a firelighter. Both produce dense black smoke with copious CO and traces of cyanide in it. I'd be more worried about the hot soot and CO to be honest.
I happen to have some rigid PU building insulation foam (ICI prototype stuff) from the late 1960's in my parents loft. I took a piece and tried to set light outside (taking appropriate safety precautions). It had one of the earliest fire retardant formulations in it (which actually makes the smoke even more toxic if it burns). The backing paper burned off and the surface of the foam charred but that was all. I was quite surprised since I was worried that the foam might have been original raw batch without any additives. If they could make PU foam resist fire at that level back in the 1960-70's I am sure they can make PIR do the same.
Snag is that the blowing agents have changed and some today may well be using pentane (which is itself a flammable gas).
However there is still a possibility of someone buying a cheap grade that is only suitable for cavity wall insulation where it is surrounded by inert brick/breezeblock walls on either side excluding the air and then strapping it to the outside of a block of high rise flats.
Structural timber doesn't catch all that easily unless you are very very unlucky (once a fire takes hold all bets are off). Paper and cardboard and thin wooden furniture are much more vulnerable. Very old foam settees from the pre fire retardant era are potentially lethal.
Now is good time to check that yours has a fire certificate stitched into the underside of the cushions (especially if you are a smoker).
It might be appropriate to ban the sale of the cheapest nastiest variant which is only safe to use *inside* the cavity of an inert brick wall or behind plasterboard (or at least add warnings to that effect).
I notice Celotex have voluntarily stopped selling their RS5000 grade as used on Grenfell tower.
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