Hi,
No built in support for this in the Xcode steps, but it’s definitely possible with custom scripts.
If you use our Xcode Test step to run the tests that exports the full raw xcodebuild output log.
The path of that raw xcodebuild output log path is exported as BITRISE_XCODE_RAW_TEST_RESULT_TEXT_PATH environment variable by the step (you can check the Outputs of the Step in the editor, it’s listed there, as well as added to the Insert Variable button’s list after the Xcode Test step), so you can do anything with that after the Xcode Test step.
Feel free to write a script which parses the warnings you want from it, then expose that as an env var (http://devcenter.bitrise.io/tips-and-tricks/expose-environment-variable/), which can then be used by any other step (including the Slack step).