Comment 10 for bug 1943120

I Tested netplan.io 0.103-0ubuntu5~21.04.2 inside a fresh Hirsute LXD container:
$ lxc launch ubuntu-daily:hirsute hh
$ lxc exec hh bash

=> Enabled hirsute-proposed & installed/upgraded netplan.io

root@hh:~# dpkg -l | grep netplan
ii libnetplan0:amd64 0.103-0ubuntu5~21.04.2
ii netplan.io 0.103-0ubuntu5~21.04.2

root@hh:~# netplan set network.bridges.br54.dhcp4=false
root@hh:~# netplan get # check that br54 is present
network:
  bridges:
    br54:
      dhcp4: false
  ethernets:
    eth0:
      dhcp4: true
root@hh:~# netplan apply
root@hh:~# ip link show br54 && echo OK
3: br54: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000
    link/ether 7a:32:79:fc:e6:a2 brd ff:ff:ff:ff:ff:ff
OK
root@hh:~# mkdir -p /tmp/np_backup/etc/ && cp -r /etc/netplan /tmp/np_backup/etc/
root@hh:~# netplan set network.bridges.br54=null
root@hh:~# netplan get # check that br54 is NOT present
network:
  ethernets:
    eth0:
      dhcp4: true
root@hh:~# netplan apply --state /tmp/np_backup
root@hh:~# ip link show br54 && echo ERROR
Device "br54" does not exist.

In accordance with https://wiki.ubuntu.com/NetplanUpdates I'm also attaching the full autopkgtest logs for those builds:
https://git.launchpad.net/~slyon/+git/files/tree/LP1943120/hirsute_amd64.log
https://git.launchpad.net/~slyon/+git/files/tree/LP1943120/hirsute_arm64.log
https://git.launchpad.net/~slyon/+git/files/tree/LP1943120/hirsute_armhf.log
https://git.launchpad.net/~slyon/+git/files/tree/LP1943120/hirsute_ppc64el.log
https://git.launchpad.net/~slyon/+git/files/tree/LP1943120/hirsute_s390x.log

The mentioned autopkgtest regression is gone after a retry.