Additionally to all of that, another idea might be to ensure that we have a convenient snippet published on snippets.xwiki.org or even tiny easily installable extension on extensions.xwiki.org so that it is convenient enough for someone using or depending on it to put it back in place once we remove it from XS.
More generally, I guess, we could say that whatever we remove (even if to the attic), should also be published and release on e.x.o. AFAIK, we currently don’t create extra artifacts (doc/release) for retired modules or components if they did not already exist, but we leave them in place if they did and link to them from the retired repo. It would surely make life better for anyone upgrading and finding out their dependency got sent to the attic, but, at the same time, would also create a bit of pollution on e.x.o with abandoned extensions. WDYT?