No regulators have made any demands for WPZ (or ETE or WMB) to dispose of anything at this point.
This entire transaction has been noteworthy for the lack of disclosure over progress. Agencies generally don't make private demands known publicly, that's the company's job. And the company has been silent.
No doubt Williams may be trying to sell other co-owned assets for a multitude of reasons not even pertaining to the merger.
Most of the large investment grade MLPs are trying to make asset sales in order to improve credit metrics. The problem with this is the most likely buyers under other circumstances all want to make asset sales at the same time. So far, the $100MM terminal sale by PAA to PBFX is the largest announced such sale in 2016. WPZ's announced 2016 capital plan includes asset sales for non-transaction reasons as you noted.