\Planning_MilestonePaneFactory, there is a bunch of code to handle "preloading" of backlog items and "preloading" of milestones.
Basically, instead of letting planning v2 app load its sub-milestones and its backlog through REST, it would pass them as JSON objects.
Unfortunately it all relies on a constant flag, set to false in both cases.
It has stayed at "false" for 5 years at the time of writing this artifact. It adds a lot of complexity (many ifs()) for nothing. Let's drop this code.