(4e) tried simplifying NPCs down to just a couple of numbers, and there was no way to reconcile that with the system in place for PCs (...) Most (2e) NPCs had no class levels, a couple of HP, and maybe proficiency with a weapon or two, because they were level 0. (...) There were no "generic" NPCs (in 2e). If NPCs were worth fighting, then they were worth statting up properly.
4e minions are more flavourful and more versatile than saying "Almost everyone is an 0th level character" while not significantly increasing the overhead in the way the 3.X NPC classes like Commoner and Expert did.
Hearing this discussion about building NPCs over the years. It fills me with
determination.
The Sliding Scale of Freeform-NPC-Building versus Structured-NPC-Building
Freeform <-> 4e <-> 1e <-> 2e <-> 5e <-> 3e <-> Structured
4e was advertised as "DMing made easy" and it followed this mantra when building NPCs. There were no "hard rules" for statting out a recurring NPC. So NPCs followed the monster rules: Either make it a disposable "minion" or grab an existing monster-of-the-same-level and refluff it. There were some basic rules to "level up" a monster, but even then the book advised it only works for like +3 levels or so, and couldn't stretch much beyond that range.
1e was (I assume) a mix of novels, boxed sets, and rulebooks. I'm sure that somewhere, in some magazine, there were stray sentences like "if you want the orc to be stronger, add 1d6 hitpoints for each extra level" or sentences like "the noble gets 1 hooker per level. Roll on the random hooker table on page x" or sentences like "if the players take too long to find the lich, then it gets 3 extra levels, +5 wisdom, and other buffs". So the 1e NPC advancement rules were loose and flexible like 4e. But unlike 4e, the 1e books gave many examples and snippets that served as general templates for advancing NPCs.
2e surely stands as middle ground for structured NPC building. The 2e books were more organized and presented rules for "Level 0" characters as Saelorn says. The structure was there, but 2e has the advantage of "CharGen Simplicity" where characters are just plain simple to build. NPCs didn't need feats or "Skills" in 2e. Backgrounds, Kits, Character Points didn't exist until later splats. 2e NPC creation was a mix of simplicity and structure.
5e is similar to 2e, as both have a structure for leveling-up your NPCs. But 5e NPCs are much more complex. NPCs need feats, backgrounds, skills, and a complex Challenge Rating. All of these are defined game terms. Whereas 4e would tell you to CopyPaste an existing monster and make it an NPC, 5e encourages you to build the NPC with quite some investment in character-building rules.
3e is the extreme case of NPC rulebuilding. Layers and layers of NPC-only classes, class templates, level adjustments, prestiege classes, spell-like abilities on top of basic skills, spells, feats and more. It should be noted that expert game designers constantly mis-stat their 3e NPCs due to the bulk workload. The main presentation of a decent-level NPC could fill two pages easily without any artwork. But the NPC would reference half-a-dozen addition sources where you need to lookup the spells, skills, classes, templates, feats, etc ... which would lead to more references. The entire text needed to display an NPC's full ability could easily stretch over a dozen pages.
Seems accurate

Right?