That article is a classic, and is used by most people I've talked to implement threading if they can't rely on the IMAP server supporting the THREAD extension [1].
However, it also serves as an example of some of the complexity that clients are expected to handle to provide a modern email experience. That algorithm isn't trivial, and reimplementing it with every client is expensive in development time, and error-prone.
As mentioned in a sibling comment, the Switchboard server's roadmap includes making it thread aware, leaving Switchboard clients lean.