But in DSL, it is one of the first weapons you see in the Weapons tab, and is perfectly legal to use, being a non-hidden part.
The flamethrower needs fuel, considered to be in CO2 tanks. So, basically, you need CO2 to have a working flamethrower.
In DSL, it is a rather efficient weapon if used right,notably suited for Pushers but has other uses as well. It is a short ranged weapon, racks up points quickly, and if aimed well, can torch the chassis easily.
It damages only the chassis, but can't kill a bot, not being able to damage the control board. You need a physical blow to finish off the bot.
The two flamethrower specialists are JoeBlo (who discovered that putting flamethrowers on flails and spinning them dramatically increases their range, and made a special .py for it) and Naryar (who made a special .py as well for flamethrower pinners)