I don't think wpa_supplicant.conf has been used for sometime, as they moved to cloud-init for bootstrapping. It requires the network-config file instead, the format of which is documented on the cloud-init documentation page.

I happen to have been experimenting with this for the past few weeks, and the most persistent issue was getting wi-fi to work correctly. It's quite a common issue, with any number of hacks. I offer my own network-config below, though I've only tested it with provisioning Ubuntu Server on the RPis so far (I have two 3B+s).

  network:
    version: 2
    renderer: networkd
    wifis:
      wlan0:
        regulatory-domain: "AU"
        dhcp4: true
        dhcp6: false
        optional: false
        access-points:
          "<access-point-name>":
            password: "<password>"

The important parts are:

1. The renderer, as the default is NetworkManager, which doesn't work correctly with RPis (at least on Ubuntu Server). It may work with RPiOS, but I haven't tested it yet.

2. The regulatory domain, the lack of which is what disables wi-fi in the first place. I forget how much testing I did with the format, but I believe it must be uppercase (I don't remember about quoting the string, however).

3. Disabling IPv6 may be relevant, though unlikely. It was just in a working configurations I found; I just haven't had time to confirm it. The relevant line in my user-data file is as follows:

  bootcmd:
  - sysctl net.ipv6.conf.all.disable_ipv6=1

The rest of the configuration is standard, though I purposefully made the wi-fi non-optional so I could confirm that wi-fi worked (my only Internet at the moment is through my iPhone hotspot, which was another source of issues, but that's a whole other story).

NB. According to someone else, the imager has the respective command line options for user-data and network-config, which I didn't know.