I finally got around to taking a look at the Big Sur outline shape, only to realize that someone else had beaten me to it. Here's what the Joplin icon currently looks like on Mac:
I actually started from scratch because I hadn't noticed the new icon. The first tweak I made was replacing the rounded rectangle outline with a "squircle", that is, the peculiar mathematical shape used by iOS 7 and later, as well as macOS Big Sur:
I based my outline on the template provided by macosicons.com. Because I was working with the existing SVG file, I just took the gradient and glyph and scaled them to fit the new clipping mask. The template included a raster drop shadow, though I subsequently replaced it with the programmatic one used by the existing icon.
At this point I discovered the updated icon from three weeks ago, and one thing I noticed (in addition to the use of the rounded rectangle outline instead of a squircle) was the use of a bevel. My understanding with the macOS icon guidelines is that pseudo-3D detailing should generally only be used when a design represents a physical object. Also the beveling in the new icon was rasterized, so I was not able to recreate in SVG. The template I used had a much more subtle (rasterized) bevel, which I dispensed with, and the old Joplin icon for macOS also had a much more subtle bevel, so bevels are not necessarily completely verboten.
Another tweak I made, also based on the fact that the icon does not represent a physical object, was to replace the "J" glyph's drop shadow with an inner shadow, based on the example set by the macOS Apple Music icon:
This is the result I ended up with:
As you can see, the differences are very subtle. (FWIW because the exact placement of the glyph as well as the exact colors of the gradient I got from the existing SVG file, any differences there are incidental. The color space might also be mucked up, idk.)
Here are the icons side by side for easier comparison:
I have an open merge request with the updated icon, though I am open to feedback, especially about the glyph's inner shadow (and any incidental changes I may have messed up). I uploaded raw vector versions in both SVG and PSD. The SVG version does not have the inner shadow because inner shadows are rasterized in SVG, which makes it un-editable, while the PSD version does. Feel free to make further modifications and upload them here or in the
mac-icons branch if you're comfortable using GitHub!