It does not seem to be open source though. The "Website" link only points back to the Chrome store and I found no mention of source code in the description. The Great Suspender at least was available as open source so one could go from there and a) notice that the release tags stopped, b) use the existing source there.

For me, it is also a superior alternative to Tree Style Tab on Firefox. It has better keyboard shortcuts, and drag and drop support of tabs or entire subtrees is more reliable than TST.

Unloading entire subtrees and restoring them on other computers is also a nice feature that I don't want to miss anymore.

The extension is closed source, and its dev sometimes needs a few weeks for important fixes, but it is still one of my most favorite browser extensions out there.