Hi Greenest,
I don’t know of any mattresses sold in the US or Canada that only contain steel springs that don’t have any foam or fiber padding in the comfort layers or the quilting in the cover.
There are some mattresses that use microcoils on top of an innerspring so there are two layers of steel springs in the mattress but they also have some type of padding above the microcoils. They are typically called “coil on coil” mattresses (see this article).
There is more about microcoils that are used in comfort layers in this article and in post #8 here and post #2 here
In terms of durability it would depend on the specific materials and components in the mattress but steel springs are durable components and aren’t normally a weak link in terms of the durability and useful life of a mattress.
Phoenix