There are a few bug reports related to the join node. It would be nice to save some time aside and see if anything drastic has to be done there.
I don't think there is too much value in having this task anymore. We don't really need to do any drastic changes to how it works now imo.
The one issue that people run into from time to time is that "legacy" attributes (vertex groups/vertex colors/vertex weights) become generic attributes in the join geometry node in some cases. However, this is not specific to the join geometry node as it also happens in other cases (e.g. when realizing instances). I posted some more thoughts in the other task some time ago already: https://developer.blender.org/T85962#1122677.