The Flashlight is an item introduced in the 2023 remaster of Quake II. It is only available for pick-up in The Reckoning single-player campaign; Joker can find it near his drop pod. Once collected, the flashlight seems to be attached to the player's person in a similar way to the flashlight from the BFG Edition of Doom 3. It can be switched on or off at any time via the inventory wheel.
There doesn't really seem to be a downside to switching it on at the beginning of the campaign and leaving it on; it doesn't interfere with the player's ability to use other items, it doesn't run out of power, and it doesn't seem to alert the Strogg to the player's presence, as it might conceivably do against a human opponent.
The Marine protagonists in the Call of the Machine expansion seem to have similar flashlights that they switch on when entering dark areas, although the player has no control over that.
In Quake I
A modification for Quake by Jeff Epler was released on August 22, 1996. This modification gives the player the ability to use a flashlight, which can be turned on and off at will by using impulse 12.
The flashlight allows the player to see in darkened areas with a lit up sprite in the location the player is aiming. As it is not a ray of light, but instead a modified sprite, there is no limit to the range. A player must simply aim at a location to light it up.
A flashlight being shone on one of the starting walls of E4M8: the Nameless City.