Source? There's very little risk of an e-bike catching fire unless you're charging the battery. Most of the risk will be at home.
Battery fires gets a lot of attention because they're a new thing, but they're far safer than ICE alternatives in almost all circumstances. At least that's what the fire department here in Norway says, and they have pretty good experience with EVs by now.
When you ride trails you sometimes fall down. Sometimes you catch a branch with your bars. Sometimes your bike lands on rocks or logs.
The down tube of an ebike frame is often where the batteries are located. This area takes a lot of abuse from the front wheel and is in perfect position to take damage during frontal impact.
Look up my YT @Platinum_Racing to see an example of terrain that could very easily destroy an ebike or gas bike.
Regarding fire I’d be worried about the hot exhaust on a dry 100+ day.
And I don't think the temperature of the exhaust stream would be enough to light a match 6" away from the tip of the muffler. You'd have to reef on the bike and park it in a pile of dry leaves up to the exhaust header for temperature alone to start a fire.
The problem is when you dump the bike. If you dump it on a tree or a rock you can fracture the gas tank (releasing fuel) or the crankcase (releasing oil). Or if you tip the bike completely over the fuel will eventually be released. Releasing these doesn't imply fire is inevitable. You still need ignition. If a fire starts you can try to smother it.
On an electric bike the same situation applies. If you dump the bike and fracture the battery (that is often attached to the downtube of the frame) then you will have a catastrophic chemical reaction. Thermal runaway is almost guaranteed. You can't even put it out by smothering it.
Furthermore, most lithium battery controllers in consumer devices are proprietary and most consumers have no idea what their true capabilities and thresholds are. Some batteries might be inherently safer than others due to simple firmware that they don't know exists and have no visibility into. There's really no hiding the capabilities of an analog machine.
And most riders immediately remove them because they gunk up with soot and restrict the exhaust.
I had assumed dirt bikes were banned on these trails due to weight and speed, leading to damage to the trail, risks to other trail users and potential structural failure of the often wooden bridges. The risk of fire with a modern gasoline engine is quite small, though air quality effects are significant.
Yes it is illegal but in the same way the 50cc mopeds are supposed to be limited at 45kph (27mph) and all the kids always modified them to go faster. I believe there has always been more unrestricted 50cc mopeds than legal ones in the street. Only some old people kept them restricted.
Most people are willing to take the risk of not getting insurance coverage and a fine once in a while to gain a few minutes per day and be the king of the road. Most only have third party insurance anyway and a bicycle or a moped don't usually create that much physical damage to other vehicules. It only becomes ugly when people hit pedestrians but thankfully and for obvious reasons it is a much rarer event than car drivers doing it.
All gas powered bikes generate heat and pollution. (dry grass + hot pipes for example)
Only defective ebikes will generate heat and pollution
However, more and more I am seeing e-bikes ripping around on hiking trails that would get me and my dirt bikes a hefty fine and impound. The risk of fire and contamination is practically the same. And these e-bikes are typically driven by the same people who would call the cops on a gas powered bike on the same trail. The irony is they ride and do peaceful yoga on trails that were made by motor vehicles whose owners were probably gentrified out of their homes. Go figure.