$json = 'твой json из примера';
$array = json_decode($json, true);
foreach ($array['data'] as $item) {
$sites = $item['sites'];
foreach ($sites as $site) {
if ($site['site_key'] === 'betfair') {
$item['h2h'] = $site['odds']['h2h'];
}
}
$result[] = [
'sport_key' => $item['sport_key'],
'teams' => $item['teams'],
'h2h' => isset($item['h2h']) ? $item['h2h'] : '',
];
}