The history of that github repo shows that counterclaims would be listed in that particular repo, along with the original DMCA request. There is no PR for a counterclaim for youtube-dl as of this comment.
So even if a counterclaim were to be filed, I would not expect it to have been filed yet.
Edit: the most recent counterclaim I see filed was done on 2020-10-08 (https://github.com/github/dmca/blob/5dfed34cdb3774fcb10cb294...)... for a claim filed on 2020-09-16 (https://github.com/github/dmca/blob/951b7ba132fd6bafab2401a7...). That's almost a month later.
Now presumably GitHub won't let you push forbidden changesets, but since hashes are chained all you have to do is modify a single byte of the root and replay everything. (I have no idea if GitHub would ban you for a ToS violation or if they would just wait for the next DMCA takedown notice to show up.)