Who develops and maintains Ubuntu's "Network Manager"?
Running apt show network-manager points you to its homepage:
Homepage: https://wiki.gnome.org/Projects/NetworkManager
Then, you can proceed to the Git.
If you type man networkmanager, you will see some links at the bottom of the page:
NOTES
NetworkManager bug tracker
https://bugzilla.gnome.org/enter_bug.cgi?product=NetworkManager
NetworkManager home page
https://wiki.gnome.org/Projects/NetworkManager
The link you found is a PPA, i.e. somebody's Personal Package Archive, which is by definition not the official place for packages. Those PPAs may be abandoned after a while which seems to be the case here.