I appreciate your consideration to revert back to the more simple keystroke approach to nudging the cursor without the key modifiers. I agree that it might be nice to use the Shift key in conjunction with the arrow keys to move the selected region, but only after the region had been defined. Then finalize the select with the spacebar as it does in 4.9.3.
I know you said that the only functional change was the addition of the SHIFT modifier. Actually, the selection process did seem to change more. If you try my steps above, one time with #4 and one time without it, you will see what I mean. In the beta, you can still use the spacebar to start the selection, but in order to use the arrow keys directly after hitting the spacebar, you now have to use the mouse to move the cursor, otherwise using the arrow keys before moving the mouse just moves the start point.
In 4.9.3, you could use arrow keys, or mouse or combination of both in succession after selecting the start point. Then finalize the selection using the spacebar again. In the beta, after starting the select with the spacebar, the arrow keys seem to only become active after first moving the cursor with the mouse. Then to finish the selection you have to hit the Enter key (not the spacebar).
If you want to select a region and move it around the screen quickly, you might consider using the Fixed region selection that can be moved freely and quickly around the screen using the mouse. It would be nice if this could be fine tuned using the arrow keys when you got close, but currently the arrow keys don't seem to have effect in v4.9.3. Maybe this could be added to Fixed Region select to make it similar to Select Region.