Fedora's telemetry is planned to be OPT-OUT
However, we also want to ensure that the data we collect is meaningful, so gnome-initial-setup will default to displaying the toggle as enabled,even though the underlying setting will initially be disabled. (The underlying setting will not actually be enabled until the user finishes the privacy page, to ensure users have the opportunity to disable the setting before any data is uploaded.) This is to ensure the system is opt-out, not opt-in. This is essential because we know that opt-in metrics are not very useful. Few users would opt in, and these users would not be representative of Fedora users as a whole. We are not interested in opt-in metrics.
Essentially they're playing with words to say it's opt in but if you just click Next like most users will do, it'll be enabled. The developer openly admits few users would opt in and complains that it wouldn't be useful.
Do you know how this will affect existing installations? Is this gnome only or any desktop?
States the gnome-initial-setup. KDE is big on telemetry only being opt-in, so it seems like just the gnome environment. Or at least I hope I'm right...
It won't affect existing installations, definitely won't. You may get a "Welcome Screen" on a new GNOME update that enables it if you just click next.
No idea about KDE, Fedora higher ups don't care about it but KDE does have support for Welcome Screens, they'll likely use that if they can.