Bryan's made some updates to the docs! Here's what Bryan just said to me (forwarded with permission):
I put the core of my narrative on wikitech [0] along with the diagram. The page I put that on could use a little more attention to ensure that references to pmtpa are gone. It might be good to remove the discussion of ceph as well since that migration (swift to ceph) was cancelled and isn't likely to start again for a while. I haven't heard anything mentioned about it in the last couple of Core quarterly reviews.
Bryan
Thanks to everyone who clarified this. I hope this helps move https://www.mediawiki.org/wiki/Requests_for_comment/Simplify_thumbnail_cache forward!