Google last month announced that it will support sellers.json on May 28. Despite the announcement, Google does not have yet the file sellers.json available on the google.com domain.
Publishers can check the Ad Exchange account settings in the “Sellers.json transparency” setting. In the setting, publishers can see the business name, business domain (if provided), and the publisher ID that will appear in the Google Ad Manager sellers.json file.
Google told publishers that intermediaries and accounts which are not directly paid by Google, in part or in total, will automatically be set as transparent and will not have access to this setting.
Sellers.json enables buyers to have transparency on the publishers that they are buying ads, and not only transparency at the domain level. Via the sellers.json, buyers can see who are the parties that are selling or reselling a given bid request.
Update (02-Jun-20): Google updated the deadline to June, 8.