> Related to that is the ability to watch games using the game-client too.

Halo 3's in-engine replay system was the high water mark of gaming for me.

This absolutely still exists - I have a library for reading Source 2 (CS2, Deadlock etc) demo files and streams (HTTP ones like CSTV).

https://github.com/saul/demofile-net

Demo files work, but I'm talking about spectating live. The "Watch" tab was removed and the ability to just browse and spectate the top games currently being played.

I'm sure the technology still exists in the engine, but it's no longer the key feature it once was. HLTV/GOTV was launched with some fanfare back in the day.

Guessing too much potential for abuse if the same server was handling both match and spectating.

Spectators don't watch the game on the same server that's hosting the game. The host server sends the traffic to a 'relay' on a delay, which spectators then connect to. Similarly for the HTTP streamed games, the game server is writing the data for spectators on a delay.

Absolutely crazy they haven’t revived this yet given the popularity of streaming.

Also allowed to watch games _live_! Long before streaming videos was a reality.

Ah, the good old days of watching live competition of quake through the game itself, chatting with others basically through the game console.

Pretty cool system.

Also allows maphacks, not cool.

I think some games allow this, I remember watching DotA 2 torunaments this way

The game engine, Source, is also using client-server architecture