Many apps and custom theme features heavily rely on alternate templates like product.pre-order.liquid or collection.json.liquid. Which can be accessed with /products/product-name?view=pre-order or /collections/collection-name?view=json direct URL.
A good example would be an autocomplete feature that uses alternate search template, which disables the global layout, but returns JSON formatted data instead.
From what I can see for now, this feature might be unavailable for the new structured themes:
When master_page_id is not blank, then pages render with the master page instead of the theme template, even if template_suffix is not blank. (https://help.shopify.com/en/themes/development/section-themes/master-pages)
Is there any replacement that would allow to conditionally switch the view for the product/collection/search pages?