I wouldn't even call that 'rudimentary' transparency support, since you're still relying on a background color to select an area of your image.

What I'm talking about is actual RGB alpha channel support

https://en.wikipedia.org/wiki/RGBA_color_space

But what you have here is still a neat function of MSPaint.exe

Be careful Transparent Selection also has a nasty bug.

- Rectangular Select part of an image

- Move it one pixel or more

- With Transparent Selection enabled Rotate -> Rotate 90 Degrees

This is the result: https://i.imgur.com/HlAkHM1.png

If you disable Transparent Selection it works as expected. I reported this to Microsoft twice over the last ten years, no fix yet...