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Amazon: Older Kindles can no longer download e-books
(www.heise.de)
This is a most excellent place for technology news and articles.
Progress sync works fine for me in KOReader with OPDS. Progress Sync Scrobble (to third-parties) is the Kavita+ feature.
My understanding was the Kavita+ items are things to do with third-party services and meta data providers that are an API/cost-based service to the dev. That being said I don't use any of those features.
OPDS doesn't do progress sync, at all... you're running something else there if that works for you.