Long ago, the guy who brought D&D from TSR to WotC (a fellow named Ryan Dancy) decided that D&D Third Edition would be an Open Source game, just like Linux and many other major software packages are Open Source.
Open Source means that ANYONE can take the system, modify it, use it, and even sell it, so long as they allow others to take their work, build on it, and do the same. They called this the "Open Game License," or OGL. The rules released under the OGL are the System Reference Document, or SRD.
That's why the entire D&D 3E ruleset is available online, and 3.5 too.
When WotC / Hasbro shifted to 4E and 5E, they discontinued this full ruleset policy. They put very little of the rules and info online. If you want to read the full 5E ruleset online for free anywhere, you can't. Well, not legally. For example, there is a Fifth Edition SRD. It includes ONE feat.
When Paizo decided to create Pathfinder 1E, they used the Open Source OGL to create Pathfinder. Because they used the OGL, they put their ruleset online for anyone to use. I have a copy on my own site.
What's more, if I want, I can add rules for liontaurs to my version, and change the rules for grappling, and add new spells, and require a to-hit roll with Magic Missiles. My right to modify the rules and publish them is contingent only on the ability of anyone else to take my rules and use or publish them.
When Paizo created PF2E, they wanted to move away from the open ruleset of D&D3E and PF1E. However, SO much of their content was covered under the OGL SRD, they had to allow use of it online. However, they chose to do so in a way that makes it difficult to understand without the purchased book. Check out the PF2E SRD online. It is very difficult to understand and use without the book.
In fact, I see that there are two versions of the PF1E SRD now online. Old New
It seems much harder to use the new PF1 SRD than it is to use the legacy version. The legacy version has text that explains how to play in general, how to go up levels, how experience points work, how feats work, and so on. All this instructional text in the new version (and in the PF2E version) is missing.
NO WAIT! I'm wrong. They just buried them! The grouped the parts of the game into sections, and listed the sections alphabetically. The Getting Started is buried on the Rules page, which is way down the list! Insidious!
I feel fiercely that rules released as system reference docs under open game licenses offer players the incredible advantage of being freely accessible -- and offer game tinkerers the amazing ability to create and recreate the games they love. Compare that to Paizo's new business model requiring a subscription and releasing only a "fast play" version of the rules on their site.
I wrote the above in October 2019, but I did not think it was enough for a screed. Honestly, I have a half dozen raw screed starts lying around the lab waiting to get ripe. (No comments from the peanut gallery, thanks!) Well, since then, I have really powered ahead to put my money where my mouth is.
Sure, I've ranted about OGL issues before. And I've certainly put some thought, lo this past decade or so, into how to make the game better, were I making it over in my own image.
Well, now I've really started down the path. I have created my own fork of Pathfinder 1.0 and D&D 3.5. I'm calling it Labyrinths and Liontaurs. My major purpose is to take all my ideas about game theory, all the needed improvements, put them in a manifesto, and design a version of D&D around them. So far I've finished the manifesto, and now I'm doing the grunt work of writing the game rules. Fortunately, I'm not intending to write the game over from scratch. I'm extensively rewriting all the classes, greatly modifying races and feats, somewhat modifying skills, and tweaking a lot else. There's still a huge majority of the rules I'm leaving untouched. But there's no reason for me to go on at great length about it here, since I have already gone on at very great length in the design notes. So go over there, take a look, and see what you think: L&L Home Page | L&L Manifesto.