A significant change in PMBOK® 6th edition is the disappearance of the critical chain method. Instead, this concept of possible bottlenecks in a process or work flow is included in the Agile sections without much explanation. When I teach critical chain method, I review Goldratt's seminal theory of constraints and explain how a bottleneck may appear anywhere in the project and not necessarily on the critical path. There are many ways to analyze and resolve bottlenecks in projects and the theory of constraints is essential in order to both understand and create a solution. Of course, it could be argued that the numerous references to assumptions and constraints is an adequate alternative to managing bottlenecks. After all, a constraint is in fact a bottleneck and normally documented in many project documents such as the scope statement. However, without a reference to critical chain method it leaves the project manager with no guidance to resolve these types of problems.
Certainly the project manager has enough to remember and students who plan to write the PMP® exam might prefer less content and theory to learn. PMBOK® is an excellent reference manual used to assist project managers by identifying processes that will help them deliver successful projects. Understanding the existence of the theory of constraints should be part of that content.