The usage of Authtokens to authenticate API calls is being deprecated on 1 March 2021 across Zoho’s Finance suite of products, which includes Zoho Books, Zoho Subscriptions, and Zoho Inventory. This change is being done in favor of switching to OAuth 2.0, a more secure and robust protocol.
This means that any of your existing workflows or custom functions, which rely on Authtokens to communicate with Zoho Books’ APIs will no longer function after 1 March 2021, unless you replace Authtokens with OAuth.