I remember that BBSes added SLIRP and PPP in order to dial in and access their Internet connection. It was mostly pay BBSes with MajorBBS or some commercial software. We played Doom that way as well.
My claim to fame was a 4-way deathmatch with Thresh [1], myself, and two other randos (like me) on Doom 2, I think the map E2M7? The rules were first to a score of 100, rocket launchers only. Final score: Thresh, 100; me, 7; other opponents -3 and -6.
The mention of Doom reminds me - games with this form of perfect synchronization allowed for VERY small replays - basically all it needed to record was keypresses and time stamps.
Unfortunately it also struggles with patching, because due to balance changes the commands don't work anymore as they are supposed to. For example back in the day Starcraft balanced Spawning Pool to cost 200 minerals instead of 150, to delay early zergling rushes by several seconds (which makes a massive difference). But if watching an old replay, it will still try to build the Spawning Pool at 150 minerals and from there everything breaks.
I recall using SLIRP back in the day with an dialup ISP that provided shell accounts in the base package, but charged extra for PPP (IIUC this was a fairly common business model). With SLIRP I was able to access internet directly from my PC without paying for PPP access.