Nixie tubes, like all gas-discharge tubes with negative light, do not use any filament.

A filament could be needed with a higher pressure gas, to emit electrons that would ionize the gas, to facilitate the initiation of a discharge. A filament might also be needed sometimes when the gas is a metal vapor, like mercury vapor or sodium vapor, in order to heat and vaporize the metal, though that is more commonly done by adding some noble gas, like neon, to cause a preliminary discharge that heats the metal until enough vapor is present, which allows the initiation of the main discharge.