[nxobject]

In the past, Valve has hired some of their most longest-tenured employees from modding, although not necessarily on GoldSrc - Counter-Strike and QuakeWorld Team Fortress come to mind. (But of course never Richochet.) The Narbacular Drop team came straight out of DigiPen with a noncommercial thesis project as well.

[bombcar]

A decent number of Mojang employees started out as Minecraft modders, and ConcernedApe hired one of the main stardew valley modders to help port code.