Why does my %appdata%\Local\Temp folder have almost 100GB in it? If this data is disposable, why hasn't Windows told me?
Yes, in %APPDATA%\Local\Temp or %temp% for short it is safe to delete files. Files that are in use and important for programs will be locked (i.e. open).
Windows does not delete files from this directory itself however if you choose "Disk cleanup" in the properties of the drive it will include a clean up of these files. (Make sure to never click "compress old files" in that dialog.)
You can also use a program like CCleaner to do regularly cleanups of this area. I use it instead of "Empty Recyclebin".