I use both depending on the device. My desktop at home and all servers use systemd-networkd and I’m very happy with it. Right now, I’m on vacation and NetworkManager comes in very helpful with the ability to quickly manage networks as a normal user with a graphical user interface.
Neither GNOME nor Plasma depend on NetworkManager, do they? Plasma will happily show information about connections managed by something else than NetworkManager, but won’t be able to manage them itself. But desktop distributions will most likely ship it as it covers basically all use cases.
What’s broken? I just added a vpnc connection on my machine (granted can’t test it since I have nothing to connect to) but there was a vpnc connection profile until I deleted it.
From my point of view, nothing else but NetworkManager makes sense to ship by default for a distribution aimed at desktop use. So I fully understand distributions doing this. My point was rather that this is not related to any particular WM / DE.
ISC DHCP Client and Relay End of Maintenance (www.isc.org)
The Internet Systems Consortium has stopped maintaining their DHCP client, which is standard on a lot of distros....