Research notes
Not found on the official Marukyu Koyamaen English online shop. Checked the full 'Matcha' catalog (all 6 subcategories, 50 total product listings) and used the shop's own keyword search for 'Zuisen no Shiro', which returned 'No Products found.' One official-domain hit surfaced in web search results: an archived/older PDF at marukyu-koyamaen.co.jp/english/catalog/Temporary_Simple_English_Catalog_for_Eng_HP_2020.10.01.pdf, which appeared to list 'Zuisen no Shiro' among older catalog items, but that PDF could not be opened/verified (fetch attempts returned 403 Forbidden and the browser only offered a file-download prompt rather than a viewable page) and it is dated 2020, so its status as a current official product page is unconfirmed. Third-party retailers (sazentea.com, narateacompany.com, wayoftea.com, japaneseselect.com) describe Zuisen no Shiro as an Urasenke-favored usucha matcha named for the 400th anniversary of Rikyu's legacy, but this is reseller-sourced information, not verified against a live official product page, so it is not included as confirmed/disclosed data here.