Is your feature request related to a problem? Please describe.
Setting up WWAN via systemd-networkd requires too much scripting currently.

Describe the solution you'd like
Having option in .network file to explicitly set IP encapsulation type for WWAN interface would be great.
Something like LinkLayerIPEncapsulation=raw-ip|802-3
The LinkLayerIPEncapsulation=raw-ip would be equivalent to echo Y > /sys/class/net/wwan0/qmi/raw_ip when this option is specified in wwan0.network matching corresponding interface.
The expected IP encapsulation setting is something which is rarely changed in a given modem setup os it makes sense configure this as part of other network parameters.

Describe alternatives you've considered
Manually scripted interface bring-up.

The systemd version you checked that didn't have the feature you are asking for
247

Note: the corresponding settings can be dynamically obtained from ModemManager so it's related to #20370.