I recently sent them an email asking.... They replied with this

You're actually one of the first to request an integration with Raindrop.io. I've noted your request in our system, so as more users request the integration then it'll move up in the queue to develop.

If you're interested, I would suggest you reach out to them for integration with us by sharing our API docs: https://readwise.io/api_deets. Our users have successfully convinced many apps to integrate with us already using this tactic!

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+1! Loved the Pocket integration with Readwise and would like to see Raindrop.io do something similar.