Hi eaesesthete,
The Seattle list was last edited in January of this year and is up to date as far as I’m aware.
Mattress theory can be very complex and there is no exact line that defines whether a mattress has what I call a differential construction or a progressive construction and to some degree this is relative to the person themselves (and how deeply they sink into a mattress). They are more conceptual than specific definitions. If you focus too much on theory rather than your actual testing and experience then you can risk becoming too analytical and end up going right past the mattress that may be best for you. They can be a good starting point in some cases but once you’ve started testing then your own personal testing needs to replace theory as the best guideline. The guidelines are best used to provide some general insights and understanding of how mattresses are put together and interact with different people rather than as a way to decide ahead of time on the specific type of mattress that would be best for you.
All mattress theory is secondary to whether a mattress actually provides you with the best possible PPP (Posture and alignment, Pressure relief, and Personal preferences) and your own experiences in actually sleeping on a mattress will always be the most important test of the suitability of your choice. Personal testing … at least as a reference point … always has better odds of predicting your long term experience than theory although theory can also be useful when you have specific reference points based on actual testing and you need to do some fine tuning in a design or mattress choice.
The 3" (or less) guideline is mostly for a mattress that uses latex in the support core because latex is much more elastic and flexible than other materials and has a greater chance of sinking into any gaps in the foundation. With firmer or stiffer materials then wider gaps are fine although narrower gaps are never harmful and are more evenly supportive even if they are not actually necessary for a specific mattress. The “best” foundation or box spring depends on the specifics of the mattress you are considering and the choice of foundation always depends on your mattress choice. There is more about matching foundations and box springs in the first two links in the foundation thread here.
The links in the foundation thread are the better “alternative” foundation options I’m aware of and are up to date (to the degree that any source of foundations can ever be up to date). My first choice would generally be the foundation recommended by a manufacturer unless there is a better and higher quality option that has similar or better support available in the links in the foundation thread that also meets the criteria of the manufacturer’s warranty. The steel foundation you are referring to uses a combination of flexible and rigid slats and would generally be a good option for for a mattress that was thinner and needed some extra “give” in some areas underneath the mattress (based on your testing) but this would also introduce a new variable that you hadn’t tested in person and in general I would tend to choose a foundation that is similar to the one that was under a mattress when you tested it. The “least risky” option is a foundation that has the same or very similar properties to the one that was under a mattress when you tested it or to the foundation that is recommended by a manufacturer or retailer if you are buying online.
If you choose a foundation that is an “active” or flexible part or component of your sleeping system (rather than just providing even rigid support for the mattress or sleeping system) then you would need to take its contribution to the feel and performance of the sleeping system and how it interacts with all the other layers and components into account in terms of PPP just like any other layer or component of a mattress.
Phoenix