Since the author of the original addon, _ForgeUser29087351, hasn't logged onto CurseForge in several years and hasn't replied to my requests for project status, I have taken it upon myself to fork his project. I credit him with almost the entirety of the code. I made a small change to remove a call to a removed API ('CalendarGetDate'). People are of course free to do this themselves, but I started this project for less tech-savvy players.

From the original project:

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This HandyNotes plugin is for the Withered Army Training scenario; please note that HandyNotes is required for this addon to work!

Features

Shows the location of the medium and large Treasure Chests on the main map and minimap, with a description of the contents on the tooltip.
Once Treasure Chests have been taken from the scenario they will not be shown; any chests recovered before you install the addon will not be shown.
Boss locations with some tips on how to defeat them.
Locked door locations and requirements

Acknowledgements

Thanks go to Tecosu for the idea for the addon and beta testing! More thanks have go to the comments on Wowhead for the Withered Army Training scenario without which I couldn't've [sic] made this addon.

HandyNotes_Syd'sWitheredChestsAddOn