It is though. I've implemented a captive portal that uses this successfully for sending modern mobile OS like Android and iOs through captive portal. Marcos works fine too, also newer windows.
To my shame, I must admit that I've implemented a captive portal before.
The article fails to mention https://datatracker.ietf.org/doc/html/rfc8952 and https://datatracker.ietf.org/doc/html/rfc8908 which get rid of most of the pain of captive portals on modern OS from a user perspective Still need to support Captive-Portal detection URLs for some edge-cases (Apple, MS, NetworkManager) and older desktops. But at least HTTP redirection becomes obsolete in almost all cases. Also has the nice feature of showing links to venue info page and remaining data volume in Android.