How would that help? The old functionality would still be around.
What would help is deprecating and, over time, removing features that, if needed, are better implemented on top of newer, better APIs. Providing source code for such an implementation would make it easy for old code to be changed to compile with the new version; all that would be needed would be changing #include <foo> by #include "foo" and changing references to the std namespace to that of the new library (there may be thorny edge cases here, but I can’t think of one)
For example, iostreams can be moved outside of the standard if an implementation of its API on top of std::format were provided.
I think they wouldn’t have to support such a library forever. A decade or two should IMO be enough. If code needs it after that, they can take on the burden of maintaining it.