Plugins for NetworkManager are added in networking.networkmanager.plugins, not in environment.systemPackages.
There was a change about 3 weeks ago that changed up how NixOS provides NetworkManager OOTB. In particular, VPN plugins were no longer shipped by default and required manual opt-in.
Announcements about breaking changes are always posted here: Breaking changes announcement for unstable - #85 by hexa. I would encourage you to set your notifications for the aforementioned thread to watching, so that you’re immediately notified of changes to unstable ASAP.