Hi there Toby! I’m happy to let you know that Customer.io does indeed have a Flutter SDK, which just released its version 1.0.0 this week, after an initial beta release: https://www.customer.io/docs/release-notes/#2023-04-03-flutter-official-release
With that clarity, I do want to let you know that In-App messaging does require the use of our official SDKs at this time, but we are looking at feedback regarding support for Segment as well. I’ll be happy to add your inquiry as feedback for our Product team on that Feature Request.
If you need any additional support as you look into our Flutter SDK, please reach out to us through the Need Help? link near the top-right of your Customer.io workspace. I hope you have an excellent weekend!