Hi Aria1778,
There is more about metal bedframes in post #10 here but the purpose of a standard metal bedframe is to raise the mattress and foundation off the floor and support the other components of a sleeping system and you would still need to use a suitable foundation on top of it because it wouldn’t be suitable as a support system for the mattress by itself (the mattress would collapse into the gaps between the metal cross supports).
When you make a mattress purchase you need a bedframe, a foundation or support system under the mattress, and the mattress itself. A platform bed that has either wooden slats that are closely spaced enough for the mattress or a solid surface would replace the bedframe and foundation combination but a wooden or metal bedframe that only has enough slats or cross supports to support a foundation but not the mattress itself would also require a foundation.
A flexible slat foundation is an “active” part of a sleeping system just like any of the layers in the mattress itself that compress or flexes under your weight so it can affect the feel and performance of the mattress. You can see more of my thoughts about flexible slat systems (such as the one that is an option with the BE mattress) vs rigid non flexing foundations in post #13 here and post #2 here and post #2 here.
The choice between a flexible slat foundation and a rigid non flexing foundation would be a preference choice based on which one is the best match for you in combination with the mattress you are purchasing. If you have the choice between a solid non flexing foundation and a foundation that uses flexible slats (with or without the ability to adjust them) then your careful testing for PPP on a particular mattress with both types of support system underneath it is the most reliable way to know which one is best for you with that particular mattress and whether there is “enough” of an improvement with the flexible slats to justify any additional cost.
Phoenix