See the attached screenshot. All the GNOME apps have scalable icons, only the IDLE icon is scaled up from 48x48 pixels.

I've found similar issue32129, but apparently the accepted resolution was to change a bit of code somewhere, not to provide higher resolution icons.

I think you misunderstood #32129.  It was specific to macOS Aqua systems.  On Windows and generally on Linux, window title bars have a little icon on the left end (at least on Windows).  IDLE uses the tcl/tk GUI framework and by default, on these systems, tk windows have a generic tk icon at the left end.  (At least that is the position on Windows.) #20406 replaced the tk icon with versions of the Python app icon.

On mac Aqua, tk window title bars do not have an icon.  Somehow Mac, Aqua, and tk interact so that the replacement icon instead replaced Apple's version of the Python app (menu) icon, which is different from the *nix/Windows python app icon. This was unintended and a bug.  The fix was to not execute the replacement code on Mac Aqua systems.

Your png shows the Python app icon used for IDLE as a Python application.  It is the same one used for Python applications on the Windows taskbar, including but not limited to IDLE.  I agreed on #32129 that it would be good if someone contributed higher resolution versions.  But it is not specifically an IDLE issue.  I will say more tomorrow.

At https://mgabrail.wordpress.com/2011/08/19/5-reasons-why-you-should-learn-python-programming/ I have found https://mgabrail.files.wordpress.com/2011/08/python.png - I have no idea what is the license of this file or where it originated from, but this looks fairly like the icon in question.

Google images reveal more liek it, usually resized or jpeged.

https://bugs.python.org/issue1490384
https://www.doxdesk.com/software/py/pyicons.html

Oh ok, I confirm that https://github.com/python/cpython/pull/17473 is an icon drew by Andrew Clover.

https://www.doxdesk.com/file/software/py/v/pyicons-full-0.4.zip from https://www.doxdesk.com/software/py/pyicons.html contains py0032.png which is the same picture (same pixels), but the MD5 is different because metadata are different.

That's the website of Andrew Clover: https://www.doxdesk.com/personal/

Copying my comment from the PR:

Could someone with the SVG also create 44x44 and 150x150 sized versions? (Like the pythonx44.png and pythonx150.png files in PC/icons.)

These are the sizes used for the tiles created by the Microsoft Store installer, and referenced from PC\layout\support\appxmanifest.py (search for pythonwx to find the current IDLE and pythonw references).

New changeset 3a69f3caeeaea57048ed3bc3051e16854b9a4cd6 by Miro Hrončok in branch 'master':
bpo-38439: Add 256px IDLE icon (GH-17473)
https://github.com/python/cpython/commit/3a69f3caeeaea57048ed3bc3051e16854b9a4cd6

New changeset abdfb3b47156a6ca5696b6f4380d412a460b718a by Miss Islington (bot) in branch '3.7':
bpo-38439: Add 256px IDLE icon (GH-17473)
https://github.com/python/cpython/commit/abdfb3b47156a6ca5696b6f4380d412a460b718a

New changeset 3a5545025685040842420c85a4a9aab5f044aeb8 by Miss Islington (bot) in branch '3.8':
bpo-38439: Add 256px IDLE icon (GH-17473)
https://github.com/python/cpython/commit/3a5545025685040842420c85a4a9aab5f044aeb8