I wonder if the whole "managerless culture" is to blame and is unfixable? (In other words, why hasn't SOMEONE had this thought about issues in N years? One is they deem it not a problem, another could be that they think to optimize for the filer, and the maintainer doesn't matter, or... there's no organization at all?)

There could be (theoretically) no one to make anyone do anything, and perhaps the issue tracker is either a quagmire of a codebase or something no one wants to touch because something else is more exciting?

That's one theory.

My other theory is they spend a lot of time on scaling problems and/or GitHub enterprise (which I haven't seen) -- and don't really do features anymore.

But it does feel there is no vision for changes to GitHub (maybe they think it's "solved") and it's ceasing to evolve in noticeable ways in any direction.

Can't really be sure. But I find it interesting. Again, the core is good. It's just curious to watch it so closely and not see the needle moving in any perceptible way.