Our Transfer data feature hasn’t been removed although it was updated earlier this year and now works a bit differently. You'd need to export your data from the trigger app, import it into a Zapier Table, then the Zap can use that for the backfill. You can learn more about that here: Transfer existing data using a Zap
In the meantime, I've shared your feedback with the Product team on making backfilling easier to action.
Let us know how you get on with transferring the data, want to make sure you’re all set!
Early access
Next Gen Zaps
Build and deploy automations from AI tools you already use. With Agentic Management, if anything breaks, the agent diagnoses the issue and proposes a fix.