Discussed by LEWG in Kona:
Do not consider option #4 (reorganize std into modules and fix freestanding in C++20).
Unanimous consent

Reserve modules with names prefixed by std, and std+(any sequence of digits) for use by the IS. Implementers may only provide modules matching such prefixes with a reserved token (“._”, depending on if that’s allowable in the module syntax) - users may not depend on those portably.
SF F N A SA
10 4 1 1 0

We eventually want both an import std and a finer-granularity modularity for the standard library.
SF F N A SA
8 3 3 1 0

For C++20, Do nothing, assume users will import . Implementers are required to ensure that existing C++ headers are modular to ensure this is portable.
SF F N A SA
0 8 3 2 1

For C++20, Modularize on the existing header structure.
SF F N A SA
0 0 2 6 5

For C++20, One Big module std.
SF F N A SA
2 3 4 3 2

Walter + Chris DiBella will write the "We reserve module names" paper.
Neil Meyers will write the "Implementers must make import <$CXXHEADER> well-formed" paper.