Entities such as grenade projectiles, dropped weapons should be visible when spectating and outside the pvs of those items

[AdamTadeusz]

Perhaps props etc should also be visible, as a spectator we don't care about the framerate nearly as much as when playing, and we want spectators to have a full understanding of what is going on in a round. If things around the players we are spectating from afar are being culled that detracts from the spectator experience

[Rainyan]

There is also the potential server-side networking impact from needing to transmit all events to the spectators, and how that would scale to a large amount of spectators. I doubt it would be an issue (would need to profile somehow I guess), but it's worth noting the performance hit wouldn't be purely client-sided in that sense.

A more conservative approach could be overriding ShouldTransmit for the gameplay-critical entities to skip their PVS checks for the observers.