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It is unlikely to appear due to iOS's lack of real background processes.

https://www.resilio.com/ is a good alternative which has an iOS app. But it won't sync until you open the app.

I'm trying this one, with mixed results: https://www.mobiussync.com/

Not available outside the US, strangely.

I found it in the AppStore searching for sync thing.

Location: Norway.

MöbiusSync is good.