C++20 introduced modules. Any symbol that is not exported in a module has module-internal linkage. While unnamed namespaces provide a mechanism to make definitions inside an unnamed namespace have file-internal linkage. Does this mean unnamed namespaces will become useless in future when modules become common practice in C++ community?
No: since (many) compilers see just one translation unit at a time, it’s still useful for optimization to indicate that an entity cannot be used in any other. It also avoids the possibility of accidental collisions between module units (even if those should be less likely than with broader codebases).