Why can we no longer drag and drop transfer/download music to our apple device from computer?

For the longest time, people have had the ability to drag and drop mp3 files from their computer file system, into their music library on their apple device, if connected via the charging cable. I've been doing this for a long time, however one day, there was no system update or anything, it suddenly no longer allowed me to transfer/download mp3 files from my computer to the apple device. I was confused, so i had redownloaded the apple itunes for windows and tried again, yet i had the same new issue. I also updated my apple device software, and redownloaded the itunes once again, however was still presented with the issue of not being able to drag and drop the mp3 files. I want to know what happened to this ability. Unless I'm the only one, I feel like most people would find this annoying, and would like to know how to resolve such an issue.