Limits

declscope reads one package at a time, and counts a use only where a name is written.

Not seen	Consequence
Uses outside the package	A scope beyond shared could not be checked, so none exists. declscope shrink loads the module to judge exportedness inside internal/
Whole-value operations on a struct	Copying, comparing or zeroing a value names no field
A composite literal of a type parameter with a ~struct term	P{1} with P interface{ ~struct{ n int } } fills the fields of whatever P is instantiated with, without naming them. A struct the constraint names, as in P interface{ Slot } or P interface{ ~*Slot }, is read
Reflection, //go:linkname, generated files	These reach a declaration without spelling it
A file that imports "C"	cgo rewrites it into a generated file in the build cache. Like other generated files, it is not read: nothing in it is reported, and its uses do not count
A declaration nobody uses	boundary needs a use to find, so unused code produces no diagnostic

For the last row, use an unused-code linter: deadcode, or staticcheck's unused for a library with no main to root from.