[minor]
1. Display underscores as spaces in menu bar
2. Allow user to run custom code

Enhancements are not usually backported.

1. The underscores are in entries such as 'minimal_hanoi' in the Examples drop-down submenu.  Changing to spaces should not hurt anything.

2. Already possible (maybe): Help => Turtledemo help has
  (2) How to add your own demos to the demo repository
which discusses requirements for turtle code run in the demo framework.   (However, users may not be able to add files to the turtledemo directory -- Demo/turtle in 2.x, Lib/turtledemo in 3.x.)  Any alternative should direct users to these requirements.

What is your proposed alternative mechanism?  Why is it better than a user copying __main__.py and turtle.cfg into their own directory of turtle scripts?  (Perhaps this option should also be mentioned as
  (3) How to add turtledemo to your directory.)

I will look at the two changes separately.  Spaces could be backported, but since they are not really typos, there is no real need to.  I will compare td for master and 3.9 to compare appearance.