ping system in games

Portal 2, Left 4 Dead?

I haven't played any of the games mentioned, so I'm not sure if we're talking about the same ping, but I remember a dedicated ping button in Starcraft 2, and not really ping, but (maybe - long time since I played those) ping + mark on map system of shouts in Battlefield 2 and Battlefield 2142 (e.g. "Enemy vehicle spotted!").

Age of Mythology from 2002 had a "ping" system.
And checking out the AoE wiki, there's a part about "The player can now signal allies which places an X on the mini map where it alerts allies of strategic locations."
So we're at 1999 earliest so far.

https://battlefield.fandom.com/wiki/Spotting
Battlefield spotting history. In battlefield games it is very difficult to play using jets without spotting function. It is because of this you won't see many jets in hardcore maps

Guild Wars 1 had a ping system too. Players could ping enemies, and click on the map to mark a position. You could also draw on the map and everyone would see it.

Obligatory "Not what you meant, but I'm pretty sure Pong had the first ping in games" joke.

off the top of my head, first game i played with markers was MAG

Age of Empires and Warcraft III are the earliest games I played that feature a ping system.