Comment 4 for bug 1870346

Many thanks for the quick answer. Unfortunately I haven't had much luck with the "renderer: networkd" workaround; I attempted the following configuration in "network-config" with a Pi4 with no ethernet connected:

version: 2
ethernets:
  eth0:
    dhcp4: true
    optional: true
wifis:
  renderer: networkd
  wlan0:
    dhcp4: true
    optional: true
    access-points:
      waveform:
        password: redacted

During boot, cloud-init printed the following warning:

Cloud-init v. 20.1-10-g71af48df-0ubuntu3 running 'init-local' at Wed, 18 Mar 2020 19:32:50 +0000. Up 12.25 seconds.
2020-03-18 19:32:50,728 - network_state.py[WARNING]: Wifi configuration is only available to distros withnetplan rendering support.
Cloud-init v. 20.1-10-g71af48df-0ubuntu3 running 'init' at Wed, 18 Mar 2020 19:32:52 +0000. Up 14.89 seconds.
...

(I can provide full logs if required)

After boot had completed, the wifi interface was down, and a simple "sudo netplan apply" brought it up successfully which rather suggests that though the config was copied in it wasn't applied in spite of the "renderer:" setting.

In general, I'm in agreement with Scott about the design of netplan, though I may well be missing some piece of information about why the current behaviour is as it is (and whether it's practical to change that behaviour).