I’m not a 100% sure but this may be related to how API authentication is changed. The API key and username parameter are now required to be supplied in the request headers. Does mailchimp support supplying custom headers to the request for RSS?
This article will help you frame requests appropiately.
Thank you, that post says that it shouldn’t change and the weird thing is when I load the URL manually it does load for me in an incognito window and it also seems to pass validation ( though with a lot of recommendations ) – Hoping I am not the only one that has had this issue…