I have made a separate bug report just now. I intended for this thread to reach every mapmaker who would be confused and wanting a quick fix for their maps before Christmas Holiday begins.
Unfortunately, this is probably needed, as one of the most well known “bug” was left unaddressed, being the object editor field “Art - Portrait Model File” being forcibly added to every custom map out there. This addition broke the unit portrait of almost every custom unit that used a model, and has not been fixed.
It is likely that this new issue of the constant “Use Relative Upgrade Costs” will also not be fixed and go unaddressed like the above constant in the map editor.
The following bug report is linked below.
https://us.forums.blizzard.com/en/warcraft3/t/use-relative-upgrade-costs-forcibly-changed-to-all-maps/34439