Man pages matching name sd_journal_include_default_namespace in section 3:

SD_JOURNAL_INCLUDE_DEFAULT_NAMESPACE.3.en	debian 11 / libelogind-dev-doc
SD_JOURNAL_INCLUDE_DEFAULT_NAMESPACE.3.en	debian 11 / libsystemd-dev
SD_JOURNAL_INCLUDE_DEFAULT_NAMESPACE.3.en	debian 12 / libelogind-dev-doc
SD_JOURNAL_INCLUDE_DEFAULT_NAMESPACE.3.en	debian 12 / libsystemd-dev