Websites no longer doing authentications for browsers running on El Capitan iMac

In the last 24 hours Reddit no longer offers correct authentication while browsing from an iMac running El Capitan. Other sites like StackExchange still work. ResearchGate.net suffered issues previously . 1. ResearchGate.net ( first to drop out... but chromium still works! as of Apr29th,2025) 2. Github.com 3. Annas-archive.org 4. Reddit.com ( as of April 2025) I’d like to learn more to understand if these failures are common or just every site gets to do what it wants. My educated guess is that this is a general web server authentication issue, if I can figure why this doesn’t work we may be able to implement a fix client-side. How can I understand these failures?