Your path forward is indeed to switch over to Incus, which you can do while keeping all your existing instances and configuration: Migrating from LXD - Incus documentation
As mentioned in Important notice for LXD users (image server), this phase out is done progressively in order to only affect users who have a migration path.
As of yesterday and until Jan 15th, those affects are users of LXD 5.20 on any of the distributions for which we have official installation instructions: How to install Incus - Incus documentation
Rather than immediately cut off access entirely, we decided to only block the less common images for the first two weeks but starting Jan 15th, all images will become available to those users. Switching over to Incus then restores full access.
It’s unclear at this point whether Canonical intends to do anything to provide equivalent images to LXD users moving forward, they may do that or they may decide that they only really care about running Ubuntu. Either way, it’s not something we have any control over.