First of all, thank you for all your replies.
The story/statement in Underscored Attributes Reference is great. But it seems to be out of touch with the status quo of Swift community.
Underscored Attributes Reference
WARNING: This information is provided primarily for compiler and standard library developers. Usage of these attributes outside of the Swift monorepo is STRONGLY DISCOURAGED.
Source: https://github.com/apple/swift/blob/main/docs/ReferenceGuides/UnderscoredAttributes.md
As discussed previously on this thread ⚠ Important packages are being released using unstable language features, many packages and end users are actually using many underscore attributes without too much worry.(Thinking it is already widely used, so the upstream may not easily just delete it.) cc @Karl
eg @_exported import @_implementationOnly import
Given the facts that so many end users(including App Store Apps) use some underscore attributes so heavily, it will not have too much room to just delete it or change it.
The only solution IMO is deprecate the old one and delete it or formalize it to the language in a next major version of Swift (Source breaking change).
But if we consider delete or modify a underscore attribute to be a source breaking change, it is very not that much different from an "official" feature.