According to the Scrum Guide, the Scrum Master has the following responsibilities(among many others):
  • to serve the Development Team by
    • "...Coaching the Development Team in organizational environments in which Scrum is not yet fully adopted and understood..."
  • to serve the wider organization by
    • "...Planning Scrum implementations within the organization..."
    • "...Leading and coaching the organization in its Scrum adoption..."

As such, when it comes to ensuring that Mitigating Practices are put into place to deal with Organizational Constraints, it is the ScrumMaster's responsibility to make sure that happens. This doesn't mean that it is the ScrumMaster's responsibility to create or even execute the mitigating practices, but it is definitely their responsibility to make sure the mitigating practices get installed.