The primary use of this seems to be checking for network availability, not anything more detailed.

A possible starting point for a replacement could be NetworkStatus in here: https://invent.kde.org/pim/itinerary/-/tree/master/src/solidextras - and as the folder name already tells you, I'd see that in Solid eventually. The API is minimal (connected: yes/no/unknown, metered: yes/no/unknown, QML compatible), backends exist for XDG Portal, NetworkManager and Android, so at least Windows is definitely missing, *BSD might or might not work via the XDG Portal API, no idea. Is that sufficient? Ok to add that to Solid?

This isn't something we have to wait with for a Qt6 port, we can already add the replacement now and move over to that. That might even bring immediate benefits for Flatpak users, as this would avoid the need of allowing access to the host NM and rather only go through the portal.

There's now a QNetworkInformation class [1]. Currently it only does "is online", no captive portal support and no metered detection. It's a start at least.

Maybe we could help upstream on that epic: https://bugreports.qt.io/browse/QTBUG-91021

I already moaned about lack of captive portal detection API in the "isHostReachable" task :)

[1] https://codereview.qt-project.org/c/qt/qtbase/+/327554 and https://codereview.qt-project.org/c/qt/qtbase/+/327993

The relevant Qt bugs were fixed, once we branch KF6 we can port to this new API.

It looks like alot of the usage of QNetworkConfigurationManager is about the QNCM::onlineStateChanged signal, which can be ported to using QNetworkInformation::reachabilityChanged in Qt6. https://lxr.kde.org/search?%21v=kf5-qt5&_filestring=&_string=QNetworkConfigurationManager

The only other usage I know of is KIO's ProxyScout, which might be ported to QNetworkInforamtion too https://invent.kde.org/frameworks/kio/-/merge_requests/677 though I am not sure yet (and can't test, no access to a proxy setup).