Every time I start FiveM, the ‘FiveM’ software (not a game) starts on Steam, and this library item looks and acts as a normal Steam game (or perhaps an external game added in the library), except being categorized as a software, and, most importantly:
- it disappears from the library once Steam is exited (PC turned off, etc.),
- can’t be removed from the library (but it wasn’t part of a purchase),
- and is states to take no space (0b).
It appears that this library item was added sometime around January 8-13 of this year, and either replaced the Source SDK Base 2007 tool (not a game) in such a behaviour, or became a second Steam library with such a behaviour.