It's possible to create an unsigned iOS .ipa without code signing.
Run the Flutter app first to an iOS simulator to generate iOS build on /build/ios.
Create a folder anywhere you like with the name Payload - note that this is case sensitive.
Navigate to /build/ios/iphonesimulator and copy Runner. app to the Payload folder you've created.
Compress the Payload folder with default .zip format
Rename Payload.zip to Payload.ipa
Installing the .ipa file to an iOS device is a different topic, however.