I have no idea the version numbers, i'm just referring to comments in the example backup_vm.py script

Note that this feature API is supported from version 4.3
but the feature is currently in tech-preview until libvirt will
release official release that contains the support for incremental
backup.
Using this example requires a special libvirt version supporting
incremental backup.

Given that previous poster also said its 'still' in tech preview on libvirt/qemu side, I assumed you might still need a special version of libvirt to enable the feature.
From: Gianluca Cecchi [mailto:gianluca.cecchi@gmail.com]
Sent: Tuesday, June 9, 2020, 6:49 PM
To: Joseph Goldman
Cc: users
Subject: [Possible SPAM] Re: [ovirt-users] Re: how to test incremental backup in 4.4

On Tue, Jun 9, 2020 at 10:39 AM Joseph Goldman <joseph@goldman.id.au> wrote:
I could be wrong but i believe you need to push libvirt / qemu to a newer release on your hosts to enable the feature. This may be a 4.3 only thing though.


I have 4.4 and currently on host:
qemu-kvm-4.1.0-23.el8.1.x86_64
libvirt-daemon-5.6.0-10.el8.x86_64

Possibly needed sw pieces will be only in 4.4.1?

Gianluca