Up until recently, some businesses have been able to pre-check the Messenger checkbox which appears under the add-to-cart button on product pages. Facebook previously allowed this box to be checked by default, but as of 9th April 2019, this feature has been removed. Note that the checkbox feature is still there, it’s just the pre-check by default option that’s gone.
Everything else to do with the Messenger checkbox remains the same. You can still get opt-ins using the discount opt-in checkbox or the opt-in model popup. (These can be set up in the “Growth Tools” section in Messenger.)