It seems like the UI and the API both support a config block at the slack notification rule level to f.ex change the channel option. However this does not seem to be supported in Terraform?
Closing this as it seems when adding multiple rollbar_notification blocks to the same project they all get merged together which covers what we wanted to do.
It seems like the UI and the API both support a config block at the slack notification rule level to f.ex change the channel option. However this does not seem to be supported in Terraform?