From my understanding this isn’t an endpoint thats being deprecated, but this behavior seems indicative of some kind of brown out? The API responses we see are:
<html>\r\n<head><title>400 Bad Request</title></head>\r\n<body>\r\n<center><h1>400 Bad Request</h1></center>\r\n<hr><center>nginx</center>\r\n</body>\r\n</html>\r\n
We create lots of certs programmatically, so this isn’t specific to a particular domain (I can often retry the same request later and it works just fine). We do not see this behavior at all in production.
This could be an indication that the code/client you are using has not been updated to [correctly] support Post-As-Get change deployed to stage (but not prod).
In that case OP would be seeing an ACME error response like this one.
That we see the default nginx error page strongly suggests that there is an HTTP protocol violation going on. But no idea why it would be intermittent.
<html>\r\n<head><title>400 Bad Request</title></head>\r\n<body>\r\n<center><h1>400 Bad Request</h1></center>\r\n<hr><center>nginx</center>\r\n</body>\r\n</html>\r\n