The following aspects should be taken into consideration:

If an assembly reference is removed, it might still be possible to compile successfully, but with different result (a different extension method is used, etc.).
Speculative binding triggered by SemanticModel should have no effect, shouldn't "pollute" the result.
A linked (NoPia) assembly is never added to the AssemblyRefs in the resulting assembly, however it should be referenced by the compilation if any types are used.
A references to a constant (constant fields/enum member) in source is replaced with underlying constant value in the resulting assembly and, by itself, doesn't add defining assembly to the set of AssemblyRefs. However, the build is going to fail if the defining assembly is not referenced by the compilation.
Similar to constants, compiler might optimize out usage of a type "used" in source (for example, an instance of a tuple is not actually created, etc.), but the type might still be required for a successful build.
Intermediate assemblies in the type forwarding chain are not added to the set of AssemblyRefs, but they should be among compilation references in order to compile successfully when the forwarding chain is used.
Some types are used by compiler implicitly without an explicit reference in source (runtime helpers, attributes, etc.). It is quite possible that compiler can silently change output when a certain API becomes available (no longer ambiguous if reference is removed) or becomes unavailable.
Compiler can embed or stop embedding some types (for example, attribute types for C# features) without resulting in a compilation error when the set of compilation references is changed.
Usage of a conditional API might not be reflected in AssemblyRefs, but the build is going to fail if the defining assembly is not referenced by the compilation.
A using/imports can refer to a declaration in an otherwise unused referenced assembly, resulting in an error if the reference is removed.
When a candidate for overload resolution becomes "bad" due to an assembly reference removal, that might cause an overload resolution failure even when the method is not the best candidate.

Provide a compiler level API to list used (or the opposite unused) references per compilation #37768

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