Yes, it's not actively developed anymore because of the large overlap in functionality with uBlock Origin as well as the maintainance burden.
However it's not equivalent, because in uMatrix you have even more fine-grained control which content a certain domain in a certain context can load (i.e. scripts, images, css and xhr requests).
Its functionality is equivalent, its ease of use isn't (at least in my opinion)
with uBlock origins UI you can block by content type and then whitelist the domains which are still allowed and that feels less granular then the uMatrix configuration dialog.
But if you check the generated dynamic rules in the settings you'll see that it supports the same granular controls as uMatrix
GP is correct afaik: the request categories for dynamic rules in ublock are image, 3p, inline-script, 1p-script, 3p-script, 3p-frame. The 3p vs 1p vs inline is kinda weird in itself since it's contextual and not relating to the content-type of the request, and we are missing css and xhr. Thing is, i realized i most of my dynamic rules are by domain anyway. Maybe if i find a real use case i'll try to look into the code and make this a bit more versatile.
However it's not equivalent, because in uMatrix you have even more fine-grained control which content a certain domain in a certain context can load (i.e. scripts, images, css and xhr requests).