The description of STABLE as a development branch to be followed with caveats, and that using it is an active process with risks you need to mitigate has been in the handbook since at least 1996 (https://freshbsd.org/freebsd/doc/commit/827b1b2b292dd02f9bc9...):

> $Id: stable.sgml,v 1.1 1996-02-11 00:16:17 jkh Exp $

> FreeBSD-stable is our development branch for a more low-key and conservative set of changes intended for our next mainstream release.

..

> If you're a commercial user or someone who puts maximum stability of their FreeBSD system before all other concerns, you should consider tracking <em>stable</em>

..

> Please note that the <em>stable</em> tree endevors, above all, to be fully compilable and stable at all times, but we do occasionally make mistakes (these are still active sources with quickly-transmitted updates, after all)

Indeed, the first step to tracking stable is to join the mailing list:

> <heading>Using FreeBSD-stable</heading>

> <p><enum><item> Join the freebsd-stable mailing list.

So the nature of the branch hasn't really changed, it's more that there's now much less reason to track it - we have regular patch releases, and the ports system no longer only supports CURRENT and STABLE.