Earthscience.SE is now in public beta, and there are questions on position of geoinformatic questions there, like this one. It seems that there is not a consensus yet on this topic. For Geographic Information Systems, it is clear that they are on topic here (so they should be asked on Earthscience.SE only if focusing on the "earthscience" part). But what about software related to other earth sciences than geography - geology, stratigraphy, climatology etc. (some examples are in the linked earthscience.SE question)? Are all of them on topic on GIS.SE, or just some of them? If they are on topic here, is there any category of such software behind GIS that should be asked here rather than there?
Even if a Question is on-topic here, it may be noticeably more on-topic (i.e. core business) for another SE site (e.g. Earth Science SE). If that is the case, I think suggesting migration to that other site is reasonable.
Likewise, if we see a GIS (our core business) question on another site (e.g. Earth Science SE) that we participate in (where it may appear noticeably more peripheral) then, on a case-by-case basis, I think that it is reasonable to suggest that it be migrated here.
If I see ArcGIS Desktop (including ArcPy) questions on Stack Overflow I sometimes comment there that they be better asked here because I rarely migrate or seek migration nowadays. I think suggesting they delete and re-ask on the other site is a better practice. Also, questions over 30 days old cannot be migrated.