We originally had a long list of compatible and incompatible Feats for use with Codex, as well as guidelines on how to use them. These are in the Legacy version of Core Rules (version 2.X and 3.X, which was originally made for OGL 3.5). We kind of integrated the 3.5 Feats with the Codex ‘Martial Feats’, with one type serving as prerequisites for the other and so on.
But last year when we re-released Codex we found that other than Pathfinder players, most OGL gamers we spoke to were not familiar with 3.5 Feats and were kind of put-off by them (frankly we got a lot of grief over it). Meanwhile most of the (small number of) 5E Feats are not even included in the SRD. So to simplify matters we removed the 3.5 Feats and all references to them (and to stuff like BaB), and the current (version 5.2) release of Codex Core Rules is basically “version agnostic” and self-contained… and is essentially meant to be easily adapted to 5E or OSR games. So it is limited to the Codex Martialis Feats.
The Quick Start has not been updated in a while and still has some of the 3.5 references in it, which may be a bit confusing.
If you want I could send you the old 2.X version of Core Rules which has a lot more of the 3.5 stuff in it. Meanwhile I will post the old section on 3.5 feats in another thread and link it here for you.