I used to hear this theory about the burned valves and took it as truth without ever thinking i through. Now I’m not so sure. I think a lot might depend on other factors.
Cars with open pipes seem to burn valves, but cars with open pipes also have other modifications intended to force more fuel into the chambers. Superchargers or turbochargers supported by larger injectors and modified mapping of the ECU to keep them open longer create a situation wherein still-burning fuel is pumped out the exhaust valves, even if the heads haven’t larger valves and the ports haven’t been been opened up to prevent restriction. If the latter mods have been sone it blows burning fuel through to the extent that when they’re run hard you can see the flames blowing out (if zoomies are the only pipes). A stock engine with open pipes still should have pretty much expended the hydrocarbons before the exhaust valve opens.
With old carburated engines there was still unspent fuel at the bottom of the power stroke and removing exhaust restriction and blowing the still burning fuel out the valve unrestricted could manifest itself as burned valves, but I’m not so sure that’s true with a modern engine.
I remain open to the arguments from both sides on this issue.