Most of the stuff on GoG should fit the bill. You still can use it as a website to shop for a game and get a installer exe for it. Without any additional launcher.
As far as documented network go, I’ve never seen more than games listing a couple of ports for servers, never anything like ips or actual network protocol descriptions. I guess anything with user hostable dedicated servers should be fine.
Problem is that those old games have very low online population, so I’m not entirely sure how much “casual comraderie” you’ll find.