Maybe slightly tangential since this is about Safari - but have 𝗬𝗼𝘂𝗧𝘂𝗯𝗲 𝗮𝗱𝘀 𝗿𝗲𝗰𝗲𝗻𝘁𝗹𝘆 𝗿𝗲𝗮𝗽𝗽𝗲𝗮𝗿𝗲𝗱 𝗶𝗻 𝗖𝗵𝗿𝗼𝗺𝗲? The declarativeWebRequest YouTube adblocker I threw together in half an hour a few months ago recently stopped working on my end too. Opting out of targeted ads serves you the bottom of the barrel, which was enough motivation to waste a day of JS spelunking (YouTube's changes…
Yay; the /player endpoint can also use XHR. Add the following before the 'return' in the JSON.parse intercept:
if (typeof data['adPlacements'] !== 'undefined') {
data.adPlacements = [];
data.playerAds = [];
}