Those days didn’t even exist (or if they did, they were very short). I feel like Steam came out just before the broadband boom (when they released HL2, it was still on five CDs). And by the time other devs started switching, most of them just went to Steam.
However, re: the broadband thing, they could distribute with bittorrent, kind of like how linux distros do it.
Uh, all games had patches and mappacks downloaded directly from the game’s distributor. It was extremely slow, hence the invention of CDNs like filehippo, hwfiles, 2cows etc and some gaming companies (at the time you could rent your own server and host the games. Incredible, huh? You could also have LAN parties and play with your clan against other clans all using LAN, no internet required) were also doing mirroring of those files (in Italy it was gamearena and ngi, for example). Steam wasn’t even a thing.