Does there exist standardized language-agnostic data structure notation?
In terms of notation:

Isn't XSD exactly what you are looking for? (barring all the &quot;severe criticism&quot; ...). Also not sure if you are looking for a notation convenient to hand-write, that's certainly NOT XML :-).
Typedefs and preserves seem to have more of a mathematical basis, perhaps closer to what you are looking for. A related HN thread.

I suspect you already know a lot of these projects, usually the focus is in serialization:

The IDL list at wikipedia ...
Amazon's Smithy and Ion.
Microsoft's Bond.
And of course Google's protobuf and flatbuffers, but also Mojo, oriented to IPC, and Fuchsia FIDL which seems similar.

Investigating this space I found Arrow, &quot;a language-independent columnar memory format&quot; for sharing memory between programs, with libraries for a dozen of languages. Turns out it uses flatbuffers under the hood.
Now that I came up with this list and I suspect you may be familiar with a lot of them, perhaps will be useful to know in which way the thing you are looking for is different. I think the keyword &quot;notation&quot; on your question is the key.
Cheers!