By support, I mean, if you start watching a series in an app (e.g. HBO Max), then it will show up in Watch Next with the next episode displayed, ready to be clicked on to watch. Or if you start watching a movie and don't finish it, it will show up in Watch Next.
I've read different things on this matter over time. I've seen reports of Netflix, Prime Video and Disney+ (perhaps other apps too) working with Watch Next, at least at some point, and then not. I'm wondering what the status of this feature is right now.
Netflix still hasn't added support, despite I've requested the feature more than once, I assume more people must put pressure on them in order to get it.
I can't speak on the rest of apps.