I was involved in the Ultima Online scene.
I submitted RunUO patches to our local forks (I then had no idea how to help with "upstream" stuff), and I also was fan of Sphere based servers.
Also I GMed a couple servers, made maps, scripts, RPG Scenes, researched lore of the series to teach to people...
One thing that happened interestingly, is that several servers over time ended rolling back expansions, even if they already had implemented them, because of how obviously bad they were for the game overall, this also affected the mainstream EA servers, each expansion made people leave instead of grow...
The most popular servers were the "T2A" servers, that emulated the expansion "The Second Age", usually with a few minor features from new expansions (like custom house architecture).
I noticed after I started to study game design formally, that the problem of the expansions was two-fold: 1, power creep, lots of it, each expansion introduced stronger enemies, harder dungeons, and better equipment... and required that you paid for them to get all that, in a obvious cash grab, but that made the game unbalanced.
And 2, and biggest problem: it shifted the genre of the game!
When Ultima Online started, it was a sort of medieval life simulator, it had farming, alchemy, baking, and all sorts of "boring" daily stuff, there was even one interview were Richard Garriot mentioned when he was walking around invisible, and saw a warrior show up near a fisherman, and out of pity drop on the ground for him lots of good equipment, and the fisherman got offended, and explained he was a fisherman, he wanted to fish, not kill, and did not wanted swords and armour near him.
New expansions routinely broke "skills" that did non-combat tasks (example: item identification), or made them irrelevant (blacksmithing... in the original game a blacksmith could make the best equipment possible, after expansions blacksmithing became useless, with the best smithed equipment having only half the power of the stuff you could find in the "end-game" dungeons).
Meanwhile they introduced mostly combat features: new weapons, new offensive magic, new combat-based "classes" (in a game that was supposed to be classless in first place), and so on...
Also expansions gimped some past features that made the game interesting, for example in UO default map, you could attack anyone that you wanted, anywhere, the only protection players had were other players, and the city guards (that were practically invincible).
New expansions instead introduced new maps that had all sorts of shenanigans disabled, and lots of restrictions, also they made LOTS of maps, and after they pissed off players with the genre shifts, it became too much map for too little people, it became easy to wander for hours before ever meeting people.
Thus, if you wanted a "real" UO experience, private servers were the way to go.