Back in November the networkd service gained support for network bridging and now with the latest code rework there is network bonding support. Network bonding is combining two or more network adapters for network redundancy or greater throughput.
Networkd's basic bonding support happened when the bridging code was refactored to have generic netdev support and then extended with bonding support after changing around a few hundred lines of code.
The network bonding support was done by Tom Gundersen and will be part of the upcoming systemd 209 release. The feature landed with this Git commit.