@Elyot_Grant The API team is aware of your challenge in regards to setting headers for making v5 calls and we all appreciate your feedback in the process. I’m glad to hear that you found a solution using api_version=5 directly in the request. Though possible, it was never added to the documentation as it was not the method in which we wanted developers to declare a version. Since this will most likely not change before the sunset of v5, I think this is your best course of action. I will follow up if the API team has a different recommendation.
You are correct that the handling of the default API version changed since the first email communication about the migration guide and we did not communicate further when the guide was updated. We did not anticipate any issues. We’re sorry to hear this had been an inconvenience, though this is exactly why we are running scheduled shutdown windows. We want to make sure we catch any issues alongside developers ahead of the final shutdown to minimize long or undetermined amounts of customer downtime.
Please let us know if you have further questions.