The SkinVectorStyleModules hook was introduced in 8b6ed4d08fa0f209fa835b906cd64c55a50cabf0, but doesn't seem to be used anywhere in Wikimedia Git per https://codesearch.wmflabs.org/search/?q=SkinVectorStyleModules.

The hook is quite generic and doesn't provide any context. If the hook is mainly intended for unconditionally adding modules to the output, then it is safe to keep in the proposed reality of T140664. But I suspect it wasn't used that way, that absence of context in this hook actually meant the caller would use global state about the page and/or the user to determine which modules to add. We should try to remove hooks of that nature, in favour of more declarative data sharing.