RssLoader that uses a webview to parse content. This will work with ATS and `Allow Arbitrary Loads in Web Content` turned on.

RssLoader.swift

@objcMembers
class RssLoader: NSObject {
    var completion: ((_ rss: String?)->Void)?
    var webView: WKWebView! = nil
    
    func load(url: URL, completion:((_ rss: String?)->Void)?) {
        self.webView = WKWebView()
        let request = URLRequest(url: url)
        self.completion = completion
        webView.navigationDelegate = self
        webView.load(request)
    }
}

extension RssLoader: WKNavigationDelegate {
    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        webView.evaluateJavaScript("document.documentElement.innerHTML.toString()",
                                   completionHandler: { (html: Any?, error: Error?) in
                                    self.completion?(html as? String)
        })
    }
    
    func webView(_ webView: WKWebView, decidePolicyFor navigationResponse: WKNavigationResponse, decisionHandler: @escaping (WKNavigationResponsePolicy) -> Void) {
        decisionHandler(.allow)
    }
}