http://andrei3ufgijpzogyai6ejcrdwevzediyzubgqiltc2k42c2zyeakhid.onion
Go to your subscriptions page, scroll to the end of the page, so it loads everything, open the Inspector, go to the Console tab and paste the code below. It should load for a while, then download a file called youtube_subs.opml
PS: I don’t remember where I stole the code from, but it works, and that’s what matters.