Man pages matching name mpi_is_thread_main in section 3:

MPI_Is_thread_main.3.en	debian 10 / lam4-dev
MPI_Is_thread_main.3.en	debian 10 / mpich-doc
MPI_Is_thread_main.3.en	debian 11 / lam4-dev
MPI_Is_thread_main.3.en	debian 12 / lam4-dev
MPI_Is_thread_main.3.en	debian 12 / mpich-doc