Thanks for your reply @reachmazharul, but this functionality is definitely part of the core Subscribe2 plugin. Here are lines 1126-1137 of subscribe2/classes/class-s2-admin.php:
/* ===== functions to show & handle one-click subscription ===== */
/**
* Show form for one-click subscription on user profile page
*/
public function one_click_profile_form( $user ) {
echo '<h3>' . esc_html__( 'Email subscription', 'subscribe2' ) . '</h3>' . "\r\n";
echo '<table class="form-table">' . "\r\n";
echo '<tr><th scope="row">' . esc_html__( 'Subscribe / Unsubscribe', 'subscribe2' ) . '</th>' . "\r\n";
echo '<td><label><input type="checkbox" name="sub2-one-click-subscribe" value="1" ' . checked( ! get_user_meta( $user->ID, $this->get_usermeta_keyname( 's2_subscribed' ), true ), false, false ) . ' /> ' . esc_html__( 'Receive notifications', 'subscribe2' ) . '</label><br>' . "\r\n";
echo '<span class="description">' . esc_html__( 'Check if you want to receive email notification when new posts are published', 'subscribe2' ) . '</span>' . "\r\n";
echo '</td></tr></table>' . "\r\n";
}