It"s not like a catalytic converter. Doesn"t get hot enough to start a fire.
The only time I saw an exhaust system start a fire was when my Dad was putting gas in a hot tractor (plowing) and spilled some on the manifold. Serious fire, but he got it out by throwing dirt on it.
My cousin used to tell me that he could light a cigarette on a Farmall manifold when plowing. I tried it on an old Ford 8N manifold while plowing. Didn't work for me, so I put that tractor to some serious work plowing. Yes, you can light a cigarette on a Ford manifold while plowing if you work it hard enough.