Facebook made a commitment they wouldn't connect the data in order to get permission from competition authorities for the merger. I don't know how legally binding that is.
Not quite. They said it's "technically impossible" and the EU fined them for $122M after Facebook did "the impossible" and started sharing WhatsApp data with Facebook.
https://www.theatlantic.com/news/archive/2017/05/facebook-re...
When The Coca Cola Company (the folks who own the secret sauce) bought Coca Cola North America (N. America's largest bottler) there was an entire floor that TCCC empoyees weren't allowed into. CCNA had bottling agreements with TCCC's competitors.
The restrictions were strict!