I find it weird that I can view the full list of what channels a user is subscribed to on the YouTube TV app, even as a guest (no log-in required), but this information isn't available on the YouTube website for some reason, whether accessed from a mobile device or on desktop, their channel subscription list were a goldmine and had lots of old forgotten channels, it's such a shame there's no way to access it with current online tools. API key authorizations and vendor lock-ins are so fucking obnoxious.
https://www.googleapis.com/youtube/v3/subscriptions?part=snippet,contentDetails,subscriberSnippet&key=APIKEY&channelId=CHANNELID