The Spring Framework issues have migrated from Jira to GitHub Issues. See the announcement blog post for details.
While the backport for SPR-15978 addressed several issues, it unfortunately missed DefaultResponseErrorHandler.hasError which should simply return false for unknown status codes.
RestTemplate.ResponseEntityResponseExtractor doesn't tolerate unknown status codes
RestTemplate does not throw exception for custom error codes
DefaultResponseErrorHandler wastes the body of a response with an unknown status
RestTemplate doesn't consistently tolerate unknown HTTP status codes