Man pages matching name std::decimal::decimal32 in section 3cxx:

std::decimal::decimal32.3cxx.en	debian 10 / libstdc++-7-doc
std::decimal::decimal32.3cxx.en	debian 10 / libstdc++-8-doc
std::decimal::decimal32.3cxx.en	debian 11 / libstdc++-10-doc
std::decimal::decimal32.3cxx.en	debian 11 / libstdc++-9-doc
std::decimal::decimal32.3cxx.en	debian 12 / libstdc++-11-doc
std::decimal::decimal32.3cxx.en	debian 12 / libstdc++-12-doc