The way I see supplemental rules is this. If it's intended to be just that, an option to make the game different or expand it's scope, absolutely, put it in a new book.
If it's functionally errata, or clarification of things that were left murky in the core rules, or even represent a paradigm shift in how the developer see the game, put that naughty word in a pdf online for free, and revise the core rulebooks!
This whole "well, we want the core books to be evergreen" didn't work for Magic The Gathering, and it's never worked for any edition of D&D ever. If you think core Rangers are crap, don't create the Gloomstalker in some book that invalidates every other stinking subclass!
If you think Sorcerers need bonus spells, don't give that to a few subclasses and not all.
If you want to fix a race/species/origin/whatever we're meant to call them now, don't do it in a book I need to buy twice.
Thank you.