I've heard that Agony Unrated never received any patches or updates that the Steam version did.

This seems to be an unfortunate trend on GOG. I have been very supportive of this platform but it seems like a lot of games never get the patches and updates that they do on other platforms. That means that the games released here are not running as well as they are on other platforms because they're lacking the patches.

Who is at fault here? I would imagine the developers but couldn't be GOG too?

My understanding is developers are the ones responsible for the patches. There could be legit reasons they didn't make it here (Steam version had bugs this didn't, etc), but I don't know.

Last patch on the Steam page is about the undated update. I don't see a ptch # on the Steam page/patch info and no version # on the GOG version.