Comment 4 for bug 1906646

This behavior is not rPi-specific and affects Netplan in general. This happens when configuring an optional wifis entry for a Wi-Fi adapter that may not be present at boot, such as a Wi-Fi dongle.

- The following user reported it on Ubuntu Desktop 18.04 and 20.04: https://bugs.launchpad.net/netplan/+bug/1951651 (marked as duplicate)
- I encountered it on Ubuntu Core 20: https://forum.snapcraft.io/t/ubuntu-core-20-boot-delayed-by-missing-wi-fi-usb-dongle-despite-optional-true-netplan-config/27403

I got a promising reply on the Snapcraft forum thread linked above (direct link to the reply: https://forum.snapcraft.io/t/ubuntu-core-20-boot-delayed-by-missing-wi-fi-usb-dongle-despite-optional-true-netplan-config/27403/5). The relevant section is:

> The optional: true is a valid flag, but it only handles the waiting for the network to come up and be configured. The timeout we’re observing here seems to come from the netplan-wpa-wlx0123456789ab.service unit waiting for the sys-subsystem-net-devices-wlx0123456789ab.device hardware to become available, though, but that does not exist if the dongle is unplugged.
>
> According to https://github.com/systemd/systemd/issues/4413 1 we might need to change the Requires=sys-subsystem-net-devices-wlx0123456789ab.device dependency to something like Requisite=sys-subsystem-net-devices-wlx0123456789ab.device (I’m not sure about the side-effects, tho).

There's a reproduction scenario in the original post on the Snapcraft forum as well.