A decision made in advance
The crucial thing about Doom's moddability is that it was not an accident, a leak, or a community achievement won against a hostile publisher. During development, id Software designed the game to be easier to modify by separating game content from the engine itself. Levels, textures, sounds, and other data went into WAD files, distinct from the executable, which meant new content could be created and shared without patching the game or writing a line of code. id also uploaded the source for its level editing and utility programs.
That is a deliberate architecture, adopted before anyone knew whether it would matter. It reflects a specific understanding of what a game is — engine plus data, with a clean seam between them — and a willingness to expose the seam. Nearly every contemporary game was a single indivisible artefact where content and code were tangled together, and modifying one meant reverse-engineering the other. id built the door before there was anyone outside asking to be let in.
The community that arrived immediately
They arrived at once. Doom released in December 1993 and immediately attracted a sizeable following creating and sharing WADs. The first third-party level editor, Doom Editing Utilities by Raphaël Quinet and Brendon Wyber, appeared on 26 January 1994 — roughly six weeks after the game. DEU's code was released as open source, so other authors published their own versions with new features, and the editing paradigm it established still underpins modern Doom editors including DETH, DeePsea, and Yadex.
The distribution mechanics are a period piece. Around 1994 and 1995, mods moved through bulletin board systems and on CD collections sold in computer shops, sometimes bundled with printed instruction guides for making levels. FTP servers took over later. There was no storefront, no workshop, no integrated browser — just files moving between strangers over dial-up, with the community's infrastructure improvised out of whatever existed.
Then the source code
id went further than the architecture required. It released the Doom engine source code in 1997 and re-licensed it under the GNU GPL in 1999, at which point the community stopped being limited to making content and started making engines. The modified versions — source ports — could fix the original's limits, add resolutions and features the 1993 code never contemplated, and carry Doom onto hardware that did not exist when it shipped.
This is the decision that turned a long-lived mod scene into a permanent one. WAD files made new levels possible; open source made new Dooms possible. A game whose engine is public cannot be abandoned, cannot become unplayable when the hardware moves on, and cannot be taken away by the company that made it. The famous observation that Doom runs on everything is a direct consequence: it runs on everything because anyone who wants it to run somewhere can make it.
The seam is the legacy
What Doom established was not modding as an activity — people had been altering games for as long as games existed — but modding as something a developer designs for. The engine/content split is now standard practice, and the assumption that a game ships with tools, or at least a documented data format, traces back through a lineage that runs directly to id's decision to keep levels out of the executable.
The counter-example is instructive. Plenty of publishers have treated player modification as a threat, shipping games as sealed artefacts and litigating against people who opened them — the same instinct that drove Nintendo to sue over the Game Genie. id took the opposite position at essentially no cost to itself and got, in exchange, a community that has kept its 1993 game commercially and culturally alive for over thirty years, producing levels for it long after every original developer moved on. The WAD format is a technical detail that turned out to be a decision about who the game belongs to after it ships.