Back from BCN I flew BCN-CDG-AMS and while this service is being offered for flights to the Netherlands, it is inexplicably not offered for flights to France despite, you know, AF flying everyone through there. I used it anyway to see what would happen and while it did not result in a boarding pass in my app for BCN-CDG, it did give me a boarding pass (with Ready2Fly on it) for CDG-AMS, albeit initially only in the AF app (the KL app refused until I was in CDG). The check-in agent in BCN never asked for my covid documentation and the gate agent doing passport checks in the boarding line at CDG did not either. Both *did* ask others coming after me for such documentation.
I should add -- BCN seemed to have a specific Ready2Fly check-in desk. I used the SkyPriority desk so I don't know what the idea of it was, but one desk had the logo on the monitors above it.