When I first started exploring the project, I did it through reading the code of the current master branch inside my local clone of steemit/steem.
That got me started, but when I wanted to find more, I first looked through the things published on the steem blockchain. Did not find much that way.
After some time, I found http://docs.bitshares.org. A very good entry point, even if not directly into the steem's core technology as it is today.
At the end, the only source of truth and documentation that I can rely on is steemit/steem source code itself. And that is fine as long as I am interested only in HF19.
That again brings us to the link you shared. Even if the Projects section is not updated, I found a lot of info inside the discussions around the commits, their pull requests, connected issues, and the different branches.
Things ware not hidden, I just couldn't find them because of my lack of understanding of the development process.
RE: Steemit vs Priorities