Honestly any rpg selling point of being modular is somewhat overrated.
I tend to agree. That’s why I don’t bill my game system as modular.
That said, I do have a whole section of optional rules that can allow it to better emulate various editions of D&D.
The default is a Big Damn Heroes (far from invincible, but a starting PC could handle a trio of city guards solo with little difficulty) with attributes assigned from an array, an option selected at each level up and tactical combat on a grid.
But with the optional rules you could start down at “zero” on the zero to hero scale (at the most extreme option a single city guard would be a life-threatening encounter) with randomly generated, class options are predetermined and the combat rules tweaked for theatre of the mind.
It’s also designed so you can just yank any species or classes you don’t want out and you won’t need to make any extra changes to the rules as a result (i.e. a game where everyone is human and there is no magic works out of the box; so does one where everyone has to play a mystic dragon).
Similarly, while I have a detailed default setting, I deliberately designed it to be only a small region (c. 100 miles across) so GMs can easily add their own details just beyond the borders (or drop their own region onto the world a thousand miles away). I’m also finishing up a lengthy section on how to put together a campaign region to help guide new GMs through the process step-by-step (including random tables if they don’t want to decide for themselves) to ensure it will have all the basics needed for a campaign (primarily a home base and enough nearby places to have adventures in with interesting things in the wilds between them).
I also left a number of things as outright mysteries with a list of possibilities for the GM to choose from. A related section had suggestions for how to tailor the setting through the use of descriptions as anything from a post-Roman collapse Dark Ages, to a modern post-apocalypse (think Thundarr the Barbarian) to a Science Fantasy world (think the Outer Rim regions of Star Wars).
Also included are suggestions on how to tailor the tone of a campaign on a few axis; serious vs. silly, linear vs. sandbox, heroic vs. horror) to match your desires and including optional rules to implement for each extreme since the default rules are mostly in the middle.
So it’s kinda modular in that one group could be playing a serious Dark Ages zero-to-hero horror sandbox with only humans, no magic, random attributes, random backgrounds and fixed class abilities while another group is playing a light-hearted science fantasy big damn heroes romp through a quest-based campaign where everything from sprites to dragons is available and they select every class and background option they gain when leveling up.
But that’s not modular in the way you’re using the term (i.e. here’s a box of Legos; you need to put them together into a setting). It’s just taking the time to lay out for new GMs how they can adjust many parts of the setting/rules to fit their particular tastes.