As stated in my second opening sentence, this issue "implements the first phase of #37892", which gives the context and specific rationale.

There are 3 separate parts to a Python Shell.

1. Code entry, indentation, display, and copying.  This issue fixes Shell design flaws that have been recognized as such for at least 15 years.  In the process, it makes Shell *more similar* to the standard REPL in important respects.  In particular, by default, compound statements will look the same in Shell as the same code entered into REPL with 4-space indents.  It will also be possible to cut and paste into an editor have it look right.

Raymond: I do not plan to retain a buggy shell mode that in #7676 you called a "major PITA" (msg215288) and "a continual source of frustration for students in my Python courses" that you were "looking forward to it being fixed."

2. Code execution.  IDLE executes syntactically correct user code so that, to the extent possible and sensible, the result is the same as if executed directly with python.  Not an issue here.

3. Everything else.  Alternate shells are intentionally difference from and hopefully improve upon the interactive python.exe REPL.

Paul: The existence of two prompts, sys.ps1 and sys.ps2 in python.exe interactive mode is a kludge needed to interface nested multiline statement Python with *single text line entry* system terminals.  I believe that most beginners never fiddle with them.

IDLE's Shell is a GUI-based python-statement-aware interface.  The python.exe executing user code from shell or editor is never in interactive mode.  For statements entered and executed, '>>>' is quite sufficient to mark code.  The sidebar should be minimal and unobtrusive and 3 columns is sufficient.  So I reject the idea of expanding it.

#37892 mentions the possibility of later adding an expanded full-line prompt for the code entry area, such as "Enter below a complete Python statement."  When we get to that, we can consider an option (for non-beginners) to customize it.