No migration. Images just disappeared one day.
One thing is that might or might not be important is that I attached a block storage when setting up Discourse, and mapped directories in app.yml like this:
- volume:
host: /mnt/blockstorage/shared/standalone
guest: /shared
Then it worked just fine for ~2-3 months or so. Then one day images just disappeared without me doing anything (except maybe rebuilding in order to update Discourse to the latest version).