wp-includes/ms-deprecated.php:156Deprecated functionality to determine if the current site is the main site.
function is_main_blog() { _deprecated_function( __FUNCTION__, '3.0.0', 'is_main_site()' ); return is_main_site();}Introduced in MU (3.0.0). Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/ms-deprecated.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.