What? Of course it is (well, it's a messaging protocol). Where on earth did you get the idea that it's some Microsoft-only proprietary technology?
No one says it is proprietary technology and yes, it is nominally standardized. It is just so complex that it is impossible to use without heavy tooling support, and this tooling is only really usable in Microsoft's technology.
So it seems that people who use MS ecosystem like SOAP ... while almost everyone else who tried it calls is over-complicated, incomprehensible and "worst dev experience of my whole career"
Again, gotta call BS here. My early career was 100% SOAP and related WS-* technologies. And it was all in Java, Apache Axis and friends. Wildly popular at the time. Give it your WSDL, out pops a nice Java client. People didn’t like it not because it was difficult to use outside of the Microsoft ecosystem, they didn’t like it because it was the concepts were difficult to understand and there were many complicated technologies piggybacking off of it. It was hard to debug issues, Visual Studio or not. If you had to do something that a generated client could do, it was difficult, Visual Studio or not.
SOAP isn't some nice standardized language-agnostic API layer.
SOAP is .NET/Visual Studio RPC. (Not technically, but in practice, due to its complexity.)
And hey, great for you. But a lot of people want APIs to be broadly compatible.