Those were the days before developers and publishers got all anti-mod in their IP. Are there any modern multiplayer games today for the PC that allow those kinds of modding. Just thinking about the effort those modders did was something.
I haven't checked the whole Nexus (mods) network to see which multiplayer games are being modded, but from what I get, 'modern' PC games are rarely "moddable" to start with, and if they are it's pretty limited in what can actually be done to them. There's various reasons behind this happening over the past decade or so (I.E. the reduction of PC games' moddability, at least when it comes to multiplayer games or modern shooters for instance). One of the reasons is because 'modern' "PC" games are Console Ports; and when developers design the game (usually first on console, and extremely rarely the other way around in modern times) they do so with the Console's interface and perspective.
And Console games are not specifically designed so that later if someone wishes to would then be able to modify [x y z] things in (be it textures, models, game mechanics, features, damage values, item stats, sounds, environments, A.I., or other Assets). And IF a Console game does have a way to modify something somehow, then it's usually very 'tunneled' and on rails, specifically made as such by the devs and is limited by them (I.E. you can modify something only from an official in-game GUI with specific controls to do so with the controller, etc [relatively modern example for a shooter would be DOOM 2016 where there's a map maker]; and can't exactly open up a console command window or can't actually modify game Assets in a separate software only to then later on come up with your "mods" for the game).
In order to have a modern PC multiplayer game that's moddable to the extent that Half-Life 1 was, or heck Build engine games were (DOOM, etc) the devs would have to design the game from (and more importantly, for) the PC platform first. Then, they would have to ensure that the structure of the installed game Assets can be easily accessed after the game installation (even if the files are compressed in a specific format, that format should be accessible with a third party tool or an official first party tool that the devs would themselves create) in order for anyone to have a go at it. Not just using Wordpad to change some text-based values (.INI or other sort of Config files), but actually being able to extract Assets (sounds, animations, models, music, A.I. scripts, etc). And when ALL of those conditions (and maybe more) is met, you have a true-to-heart genuinely 100% PC Game, made from a PC and FOR the PC platform where you can, if you want to, access any one of the game's assets and see what can be done with that.
So, the Build engine scenario (DOOM mods), Gold Src scenario (Half-Life 1, TFC, Natural Selection and the likes, using that engine) and the GameBryo / Creation engine scenario + the Creation toolkit (Morrowind, Oblivion, Fallout 3 and 4, Skyrim) were all made under those contexts. Well, in the case of the Elder Scrolls games, starting with Oblivion, they were made for Consoles first (see the GUI on the vanilla PC version, they were console ports and that is 100% clear) but at least they DID leave the whole Assets part of the PC port completely intact and accessible (barely anything was hard-coded and non-accessible), and they DID continue to support mods (officially) by providing the necessary software to modify the game assets, so credits where due despite the fact that they did develop their games for Consoles first (after Morrowind, that is).
There's more to it, too (outside of the engines / assets stuff). But it's sort of the gist of it. But there's also plain and simple different philosophical approach that can "hinder", or rather can just stop modding from happening at all for any particular game or franchise. If the publisher is "against" modding in general, they just don't allow their devs to ever release any form of software to mod said games to start with; and beyond that, they ensure that the assets that are installed client-side are hard-coded to Hell and beyond and ONLY the devs can change whatever it is could be changed (beyond just the basic text-based Config file values that most games can still allow us to do on the PC platform to this day).
And this exact scenario (of the "modding is Evil and needs to stop" mentality from certain companies and corporations) is exactly the one thing I fear is going to happen to Elder Scrolls 6. That it may well be the first ES game (since Morrowind) that won't be moddable on PC (because they won't make a "Creation Kit" style tool for it to start with). It's also why a popular and known game like Overwatch doesn't have community-made mods OUTSIDE of the game client itself and launched by the launcher (I.E. need to have a real account, log in to the Battle.net launcher, launch the game normally without any pre-made mods done to the files, etc).
Using the example of Overwatch, since relatively recently the devs DID allow the players to go in their official game mode called the Workshop, where access to certain types of scripts can allow a certain level of "moddability". However, it is limited, and is only related to actual game-play, rather than related to modifying game assets per se. So that's why you wouldn't browse online and come up with something like a full overhaul of "Overwatch", with a different name, and it being playable offline entirely, and being a modified map that extends 5 times past the default stuff because some modder in the community would have been able to do just that to start with. And the same applies to Diablo 3, because always-online, because no access to full game assets... because, basically, we don't have the "permission" to do it, because no tools to do it anyway to begin with... and that's because the company behind just does not allow it and don't want it to happen, period.
So yeah, changes in development and gaming industry mentality at large over time didn't help to ensure that games on PC could be modified at any extent by the player / community if they wish to. It does still happen here and there but has been on the decline for a very long time coming from the "mainstream" developers in the industry. Nowadays, "PC Games Modding" usually happens mostly coming from Indie developed games because they simply have a more open mentality on the subject or they understand that it helps maintain a bigger replay value for their game if they allow it. The best "modern" and known example I can think of where modding IS possible and does allow for quite a good amount of changes with access to the game's assets is none other than No Man's Sky.
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