Interstitial
An interstitial is a page or overlay served in place of the content that was requested, which the visitor has to get past before continuing, and which an automated client generally cannot.
The kind that matters most here is the bot protection challenge: a page asking the visitor to enable JavaScript and wait. To an agent that is unanswerable, so the storefront behind it is never seen and every other quality of the store becomes irrelevant.
Marketing interstitials, age gates and region selectors have the same shape and a smaller blast radius, because they usually sit further into the journey.
Why it matters to a store
This is the earliest and bluntest way a store disappears. Unlike most failures it is total rather than partial: nothing about your catalogue is measured, because nothing about your catalogue was reached.