Comment by the__alchemist

Most of the box springs sold today have the box shape, but don't contain springs. They just put slats on the two sides of the box, encased with a fabric. That's why it is better to go with a sturdy frame with slats less than 3 inches apart due to the geometry of the coils in mattresses.

As mattress companies want to cut down the costs, they find cute names to replace the real springs. Leggett & Platt's Weblok is the spring version of foundations: https://beddingcomponents.com/weblok Other foundations LP sell have 'torsion springs'; they are not springs at all. That's why mattress manufacturers just make boxes with slats; if the boxes are thinner, they are called "bunkie boards".

Only luxury mattress makers sell real box springs along with flippable two side mattresses. In the states, at least you see Shifman. Duxiana combines two layers of springs, the bottom layer acting as foundation, the top layer allows zoned flexible springs; a topper sits on the top of these two coil layers.

Even some European mattress makers follow another approach: 5 inch Bonnell/open coils + 5 inch pocket coils + 2 inches of comfort layer--all in a single mattress. Saatva Classic Mattress follows a similar pattern; however they use cheap coils in both layers.

Historically, springs started as box springs (a separate layer); on top of these boxes, one used cotton/wool mattresses. Remember rope beds (you can see them in the third world). Folks just use cotton/wool layers on such rope beds. Instead of rope beds, one uses box springs on a frame. That's an easy replacement. Over the time, spring layer and comfort layers are stitched together. Flippable mattresses need real box springs as well.

European slats (curved slats sold at IKEA for instance) also function as springs for all foam mattresses (latex or non-latex). This is why many local mattress manufacturers in the USA don't recommend European slats for their hybrid mattresses.