Earliest examples I am personally aware of are the shareware and “mission pack” model of the 1990s. DOOM basically was given away for free with the idea that you play Episode 1 and then send iD money to get the disks of Episodes 2 and 3 in the mail. Not sure if they did a BBS/FTP server for that (or Episode 4), but Star Crusader is very much a game that even did the in game advertisement of their DLC. I have fond memories of working my ass off one summer so I had enough money to mail a check to some random dude so he would mail me back login information so I could download the mission pack and see what happened to Roman Alexandria when he was abducted by that ship.
The “expansion pack” model that people praise was the exact same thing, just using store shelves to distribute. Because games were big enough that it was viable to expect players to drive down to Radio Shack and pick up a copy. And the main issue with that is that it meant we needed MUCH coarser grain content because it needed to justify a physical disc printing and distribution. Everyone rightfully shits on Oblivion’s horse armor. But the actual DLC burst that Oblivion had was… not horrible. Whereas the Fallout 3 DLC model was downright amazing for giving us 3 (5?) different mini areas and quest chains. No one DLC was a full expansion (well, people say The Pitt was but they are idiots) but it meant that we got the kind of variety that we had all been wanting ever since we got tired of spending two hours in a single biome in Diablo 2.
Which gets us to where we are now. Some studios do dogshit DLC. Others do good. The Larian model has generally been to NOT do DLC but to instead release an Enhanced Edition one or two years later. I assume with the licensing and the added funding for BG3 they don’t think they will need to but, time will tell.
But, personally? I would LOVE BG3: Throne of Bhaal. Not so much whatever the shitty BG1 expansion nobody played was.
*: and… there are a lot of arguments that stuff like basically every ARPG and RTS expansion pack were overpriced as hell by modern standards. Which is funny since those are the ones that are most praised.