The built-in Python sync service has been removed with the Immich v3 upgrade. It was already deprecated in favor of Immich Sync Service, a standalone Rust rewrite that talks to the Immich API instead of the database and works with any Immich deployment.
On upgrade, the snap cleans up after the old service automatically: the custom database tables and trigger it created (assets_delete_audits, assets_filesync_lookup, and the delete trigger on the asset table) are dropped, and the old sync-enabled and sync-delete-threshold snap configuration options are removed. Your synced photos are not touched.
If you were using the old sync feature, migrate to immich-sync. It covers everything the built-in service did, with more control over deletion policies, and it can run on any machine that can reach your Immich server.