In today's updated official support document, Facebook says the Discovery tab will be removed from the Messenger application in the coming months. Thereafter, in Messenger, there will be other ways of interacting with the enterprise.
Explaining why the Discovery tab was cancelled, Facebook explained: "We want to make users more seamless with businesses on Messenger. We will invest more money in developing tools for connecting people and businesses, including updates to m.me links, web plug-ins, access to various portals of our family applications, and various advertising products.
With the latest launch of new enterprise tools, Lead Generation Product will be released as a Messenger template containing Facebook Ads Manager. This template enables enterprises to create an automated experience, help identify potential user groups in Messenger, so as to continue to communicate in this communication application, and can use existing CRM tools to further track users.
This feature has been in the testing phase since F8, but will be released publicly now.