It hadn't had a stable release for a year at the point when the repo was archived, because it's more-or-less feature-complete and has no major bugs. 90% of the open github issues [1] were enhancement or documentation requests.
No updates is not necessarily a bad thing. Sometimes things work well enough to leave alone.
I use umatrix all the time and there are multiple sites where it doesn't work. I think part of the problem is that not all requests are actually shown in the drop-down for you to enable them. Maybe a popup or frame causes umatrix to miss it etc. For a blocker where the default mode is "block everything", it makes sense that this failure mode would be the dominant..
Have you used the built-in logger to confirm that it's actually uMatrix and not another extension? I typically have this issue and then find that the fault was with a uBlock filter.
