Or in other words: it's now entirely safe out of the box to run NM and systemd-networkd side-by-side and be sure that networkd will manage the interfaces of systemd's other components, and NM can do its own thing, without any ownership issues.
At this point at least NetworkManager also respects the NET_MANAGED_BY= property. And we recommend that packages that allocate their own network interfaces (bridges, veth tunnels, dummys, tap, tun, …) start shipping .link files that match them, …