The workbench.library should be included with AmigaForever 2016 as it's used for their 3.x ROM and 3.1 A4000T Rom builds which don't have it in the ROM.
In a careful balance of preservation, restoration and innovation, features of all Amiga Forever Classic Support editions of Workbench 3.1 include:
Dates were preserved, while file attributes and comments were restored (relative to the distribution by ESCOM and Village Tronic)
Updated C/Version command (Y2K patch)
Addition of Libs/workbench.library (for A-4000T 3.1 ROMs and 3.X ROMs)
You may want to look at a 4000T system setup to pull the file.