How has USB File Transfer changed in Android changed over the years? In Windows

I remember many years ago when I got my first android that USB connection seemed to work more like a flash drive. You could do multiple things at once without it saying the device was busy. 

In all newer android versions I've seen I get an error saying the device is busy if I'm copying something and trying to browse files at the same time. Also I've noticed newer android devices seem to be slower with file transfers or even generating all the files in a folder.

So my question is what changed exactly and why was it implemented?

Bonus question: Can you give a detailed history on what happened with android USB and with what versions of android did it change with?
You might be referring to the USB Mass Storage Mode.


  USB mass storage is the standard protocol used by flash drives,
  external hard drives, SD cards, and other USB storage devices. It also
  was used by Android devices and even many basic phones.


At some point most  Android developers decided to discontinue with this handy feature. They have legitimate arguments or at least justifiable reasons against the feature. It was discontinued sometime since Android 4.3 and later versions don't have this feature.

As one reddit user explained the reasons behind such discontinuation:


  
  Earlier Android used to have /data partition for app data and
  /sdcard for movies, music and rest. Say you are an app freak and
  have installed lots of apps, what happens in /data fills up but still
  you can't use space from your /sdcard as it's a separate partition.
  
  Now what Google has done is used unified file system. So that you can
  share space between /sdcard and /data. USB mounts storage on host
  and gives entire block level access to the host. If this is done the
  partition is unmounted from your phone. It was fine when /sdcard was
  separate now since both /data and /sdcard reside on same partition
  Android can't afford to allow full block access to another host. This
  is solved with MTP.
  Another reason, Microsoft holds patents for long file name support on FAT filesystem. That's one patent for which they demand royalties
  from OEM's. If you use USB mount, you need to format your /sdcard as
  FAT because most users use windows. If you use FAT, you need to pay
  MS or risk a lawsuit. With MTP, Google can use ext3/4 btrfs whatever
  they please and there is no risk of lawsuit.
  Finally, formatting filesystem with ext3/4 instead of FAT gives lots of read/write improvements since Android uses linux kernel.
  


The new options, Media device (MTP) or Camera (PTP) mode, are either unavailable to earlier editions of Windows and particularly difficult to use on other OSes e.g. Linux , or too limited in terms of functionality as well UI feel.

Credits

USB Mass Storage on Android Devices