Hi Pegabo,
All warranties cover indentations that are deeper than the warranty exclusion and they should be able to provide you with a written copy of their warranty that will tell you exactly what is covered and what isn’t and the depth of any visible impressions that are covered (and any other exclusions as well).
The real issue is that warranties don’t cover foam softening where there aren’t any visible impressions (or the visible impressions are less than the warranty exclusion) and the loss of comfort and/or support that goes with foam softening can mean you may need to buy a new mattress even if any visible impressions aren’t enough to trigger a warranty claim. There is more about the many variables that can affect the durability and useful life of a mattress relative to different people in post #4 here and the posts it links to.
Phoenix