The technology, the UI, the UX, how the synchronization happens (using CouchDB/PouchDB now), it's now also possible to import a CSV from the iOS app, expense types now show global counts and costs, and there's auto-categorization (matching previous expenses with the same name).
Yes, the format is the same.
You have 3 options: pay me, use a cloud service provider, or self host.
2. Cloud Service Provider: Cloudant through IBM Bluemix. It's free for quite a bit of data.
3. You can also self host via Bitnami.
For options 2 and 3, you create an "expenses" and a "types" db inside your CouchDB dashboard, and then paste the URL with the credentials in the "URL" field inside the Settings tab (like "https://user:firstname.lastname@example.org").
Yes! You just need to use the same URL for the CouchDB server.
Like most things I do, this is open sourced in GitHub. Feel free to create issues and submit PRs.
The macOS version is still available through the old page for now, but because the iCloud sync method used in them has been deprecated by Apple, the iOS version is gone, and the macOS one stopped working in macOS Catalina.
I've tried updating Oikon 2 a couple of times but it would require a complete rewrite, so I likely won't do it, because I've got a similar, but different app that's frequently updated, called Budget Zen. If Oikon 2 stopped working for you, let me know and I'd be happy to give you a code to get it for free.