The error is for
CesiumForUnityNative-Editor.dll is a Windows DLL. It’s not loaded on macOS at all. The error message from Unity says “DllNotFoundException: CesiumForUnityNative-Editor assembly”, but that should not be interpreted to literally mean it’s a .dll file. The exception name comes from .NET’s Windows-specific history. The relevant file on macOS is either
Editor/x86_64/libCesiumForUnityNative-Editor.dylib, depending on your processor architecture.
I’m rather surprised that Cesium isn’t tested on an OS version that is supposed to be supported
Unity 2021.3 supports 10.14 (probably because it’s several years old), but Apple doesn’t. According to this site, macoS 10.14 has been obsolete since 25 Oct 2021. Only a small fraction of our users use macOS in general. As far as I know you’re the only one using 10.x. (11.0 is three years old already).
As a former Apple dev, and every other company I’ve stepped foot in that ships software, there is a room full of older machines and OSes to test and QA on.
I think you might be overestimating the size of Cesium. We try to test on a variety of hardware, but that definitely does not include obsolete versions of macOS right now.
Look, the truth of the matter is that Cesium for Unity is a free and open source product, and there are a huge number of things competing for our development time. Something like this, where we don’t know what the problem is, we don’t have the hardware to test it, it’s an operating system that isn’t even supported by Apple anymore, and there is exactly one user running into the problem? We can’t justify much time on that. Would you?
That said, I’ve asked Joseph on my team to take a quick look at it to see if there’s something simple we’ve missed when we added what we thought was the necessary compiler option before.
But the beauty of open source is that you don’t need to rely on us. If this is important to your business, but not so much to ours, then you can invest the resources into fixing it yourself. And then we would almost certainly accept the PR so that you don’t have to maintain a fork going forward. If that is somehow not worth it and switching to another product is a better alternative for you, I’m very surprised, but I understand.