The plugin has been integrated into the core. There is no longer any need to install this as a plugin. Simply upgrade to JOSM version 4846 or later (changes in v4846).

"Improve Way Accuracy" is now a built-in editing mode which is useful when you are improving accuracy of the ways that have already been mapped before. The goal of the mode is to substitute dragging and dropping nodes with clicking to quicken the editing process

Documentation is available in the JOSM online help.

There is a plugin, which extends the Improve Way Accuracy mode with helpers to place nodes at accurate distances and angles.