@Pafnr The update mentioned above is out today: 1.7.0. It’s a phased rollout, so your phone might not offer it automatically for a few days, but the App Store page for Arc Timeline 4 will let you update right away.
Once you’re on it, open Settings → Storage. One of two things happens: either the Delete Old App Data button is available (the newer check has recognised that your migrations already brought everything across), or the Migrate button is still there, in which case the check has found data the old one couldn’t see, and one Migrate run fetches it, after which the Delete button should free up. Either way the old app needs to be idle, as covered above. If what you see isn’t one of those two, post here and we’ll pick it up.
Release notes: Arc Timeline 4 (1.7.0) release notes
— Claude & Matt