To enable HiDPI on Big Sur (11.0.1):
You need to get all the files/folders that are in /System/Library/Displays copied over to /Library. Then you can copy your resolution .plist file.
To make things easier you can just copy/paste this command into terminal:
sudo cp -R /System/Library/Displays /Library/
Now copy over your generated file. Here is an example:
sudo cp ~/Downloads/DisplayProductID-413a.plist /Library/Displays/Contents/Resources/Overrides/DisplayVendorID-10ac/DisplayProductID-413a
Hope this helps!