Most of the time, adding stuff makes games more complex, while removing unnecessary parts makes them simpler. One example I like to mention is the "slow" tag in B/X weapons*. B/X is such an elegant game that it's hard to find things to simplify, but they're there.
* The "slow" tag serves no purpose, and in fact it makes weapons LESS balanced by making a battleaxe even worse when compared to a sword (notice that dwarves CAN use regular swords so they'd have little reason to ever get an axe), in addition to making quarterstaves less useful and less realistic (they are not a particularly slow weapon). Another example is Holmes Basic, which makes things even worse, causing the heaviest, most expensive weapon to be the least useful. More about that here.
Since I like elegant, almost minimalist, games (especially B/X), I'm very careful when adding new stuff, and most of my house rules are meant to make things simpler (e.g., uniform XP tables).
However, there are times when adding stuff makes things simpler. Well, maybe not "adding", but expanding, streamlining.
Skills are one example. The topic is heavily debated in OSR circles, but I think most people will agree with this example:
B/X Halflings can hide in the woods with 90% chance of success. In dungeons, they can hide in cover or shadows with 2-in-6 chances. Thieves, OTOH, have 10% to 99% chance of hiding in shadows. So, for a similar activity, we have two different systems (1d6 and 1d100 - for the same class!) and two different "progressions" (static for the Halfling, progressive for the thief).
A single "stealth" or "hide" skill would make things much simpler - and also instantly resolve the age-old question of "what if other PCs want to hide"?
Lamentations of the Flame Princess (LotFP) was one of the first games I've used that did exactly that (using 1d6 for everything). My own Dark Fantasy Basic uses 1d20 instead of 1d6. Using only d100 would work equally well.
TBH - "streamlining skills" is the minimum I expect from a Basic clone (here are some).
And giving EVERYONE skills allows you to build your own Ranger, Bard, Thug, etc., WITHOUT adding different classes. A ranger is a fighter skilled in nature, etc.
The same probably applies to feats. I counted 35 "classes" (in fact, just selections of feats to differentiate the fighter-ranger from the fighter-barbarian, for example) in Old School Feats. It has only 21 pages.
Compare this, for example, with rangers (and halflings) that have their own special abilities, saving throws and XP tables.
(In other words - sub-classes ALSO make the game simpler when compared to many distinct classes).
This is all more or less obvious. The reason I'm writing this is that I wrote a brief sketch of my Minimalist OSR and realized that, even in a game with fewer than 20 pages, adding skills makes things a lot simpler.
Anyway, take a look at the link. It is a work in progress, but I'd love to hear your opinions in the comments below!