D-Bus 1.3.1 (2010) introduced the standard "PropertiesChanged" signal
on "org.freedesktop.DBus.Properties". NetworkManager is old, and predates
this API. From that time, it still had it's own PropertiesChanged signal
that are emitted together with the standard ones. NetworkManager
supports the standard PropertiesChanged signal since it switched to
gdbus library in version 1.2.0 (2016).

These own signals are deprecated for a long time already ([1], 2016), and
are hopefully not used by anybody anymore. libnm-glib was using them and
relied on them, but that library is gone. libnm does not use them and neither
does plasma-nm.

Hopefully no users are left that are affected by this API break.

[1] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/commits/6fb917178aa19c61e909957f5146aa4565e0cb2f