Hi!
Regarding your first question: yes, the deprecated API option will remain available until further notice. That said, it won’t support new changes, and certain updates to your app may break it. We strongly recommend aligning your backend with the new infrastructure when possible, it’ll save your future self some time 🙂
Regarding your second question: you don’t need to rebuild existing automations at this stage. As long as you continue to support the legacy feature (which will remain editable until further notice), any automations already built will continue to run. However, this does mean you’ll need to maintain support for both infrastructures on your end.
If the migration is successful and you have the capacity to rebuild the automations, doing so would eliminate the need to maintain two infrastructures and reduce the effort required to migrate running automations in the future.