msvc added 128 bit numbers(in the header __msvc_int128.hpp). I know multiprecision uses __128 to improve performance so why doesn't it use msvc version?
it's seems easy to change and will probably improve performance.
As far as I know it’s not official supported. If you look through their open GitHub issues they are working on performance improvementsnow: https://github.com/microsoft/STL/issues/2520. Once it is official there is no compelling reason not to use it.