I have the same conclusion as Sam — I don’t think Yaru should be trying to apply shapes to app icons — but for completely different reasons.
The Ubuntu desktop is very much part of the Ubuntu business: as Sam wrote, “Ubuntu is a serious project”. Besides the OEMs shipping it — and besides the businesses, educational and government institutions, and non-profits that use it for workstations — most people building snaps, charms, ML models etc on Linux are doing those things on an Ubuntu desktop.
And even if none of that was happening, the vast majority of Gnome users are using Ubuntu’s version of it. This was true in 2009, it was true in 2014 (with Unity on top), and it’s true in 2019. This is why, as I said in October, “The icons that Ubuntu ships matter far more than any other OS’s icon theme (and far more than the upstream Gnome theme)”.
Now, some people just don’t know this. And maybe some people would like to think it isn’t true. But Gnome is the only project I’m aware of where people treat “upstream” like it’s a commandment on a stone tablet, where diverging is a heresy. For example, consider the Linux kernel: you can run a pure upstream kernel on Ubuntu, but nobody would seriously suggest that Ubuntu doesn’t “fully embrace Linux” merely because it doesn’t ship the mainline kernel by default. Nobody condemns the Ubuntu kernel because it isn’t “upstream”. That is not a serious criticism. It’s only with Gnome that people try to make this criticism with a straight face.
On the contrary, I think a consistent icon shape can only be solved by top-down modifications and the platform exerting itself. One reason is that some app developers or icon designers will disagree, or not know, or not care about Yaru. So leaving it up to them guarantees inconsistency.
The other reason, though, is that doing it in the platform is the easiest way to allow future changes. For example…
…When anyone upgraded from iOS 6 to iOS 7, the shape of every app icon on their device changed slightly. They were all consistent beforehand, and they all changed, and they were all consistent afterwards, without app developers needing to do anything at all. Because the shape wasn’t baked into the icon provided by the publisher, or provided by an icon theme — it was a mask applied by the OS.
If you decided to tweak Yaru’s corner radius, or tweak its aspect ratio, you’d have a much harder job, because the shape is applied to icons individually. Some app developers wouldn’t bother at all. Others would have shaped their icons a year ago, but they wouldn’t know or care that the shape has changed, and now you have a mess.
As I’ve said before, I’m not a fan of shaping icons like Yaru does (and especially not for things that aren’t apps). But if that’s your preferred design, it will be more consistent to do it in the platform — in GTK and Gnome Shell — than in an icon theme.