I get the reason for putting Arduino15 in User/appdata/local/... but I use a cloud service to sync my MyDocuments folder over several computers.
It would be convenient to move the functionality from the Arduino15 folder to a folder in My Documents.
In this way, when I make a change on one computer, the cloud syncing service would automatically make that change on all other computers.
I thought about making a link from appdata/local/arduino15 to a folder in my documents on all computers, but it would be easier if I could call Arduino with a change in the command line or config file.
It will be possible to use any arbitrary custom path in Arduino IDE 2.0.0. With the classic Arduino IDE, you can either use the default location or use the Arduino IDE in portable mode, in which case the portable subfolder of the Arduino IDE is used. Since you can install the Arduino IDE anywhere you like, this does give you quite some freedom in where the Arduino15 folder is located, if perhaps not so much as you would like..
I would be interested to know how to set the custom path in 2.0.0. I have not seen where that is possible. After installing, I saw that it uses the Arduino15 path.