Dave Grochocki, on the Microsoft Windows blog:

In this update, we’re excited to introduce two powerful enhancements that bring more flexibility to your creative workflow. First, we’re introducing project files, you can now save your creation as an editable Paint project file and seamlessly pick up where you left off. Once you’re ready to save your artwork, go to the File menu and select Save as project. The File Explorer will open, allowing you to choose a location and save your work as a .paint file. When you’re ready to return, simply open the file to launch it in Paint, right where you left off.

We’re also adding an opacity slider, giving you control over the transparency of your strokes with the Pencil and Brush tools. To get started, select either tool and use the slider on the left side of the canvas to adjust the opacity to your desired level. Try experimenting with both the size and opacity sliders to create smooth blends and layered effects that bring your artwork to life.

Better late than never I suppose.