Despite there being no official releases in years and being abandoned by its official developers, Pentadactyl was kept afloat by its users, from whom you could still get fixes and releases through google code and google groups. I also fixed some things for myself, based on help from the community. Despite all this, it did finally completely break recently (before FF 57), and I just had to not update FF for a while until I found an alternative. I used PaleMoon for a while, but now I'm switching to Qutebrowser.

"it's hard to justify holding an entire browser back just to maintain compatibility with an extension that has been abandoned by its own maintainers"

Pentadactyl was far from the only extension that Firefox decided to break. Many other extensions were affected.

As far as Pentadactyl itself went, the reason its maintainers abandoned it was because Firefox kept breaking compatibility with it over, and over, and over, and over. They understandably got frustrated by that and moved on to other things.

Then, when Firefox put their foot down and announced that they'd be changing the browser so that Pentadactyl will never again be able to work on it, no matter what its developers did, a lot of people didn't see the point of putting more effort in to. It survived only because of the dedication and help of the remaining community. Now there's nothing even they can do. Not with Firefox anyway. So we're moving on to something else.