Jane Yeah - Bummer.
If you now go the next step, you risk a strike. 3 strikes and YT shuts your channel down.
That's all about the bad news.
The good news is, a strike disappears after 3 months automatically - no consequences. If you are not dealing with YT-strikes too often, this is indeed a risk worth considering - as I said, even a strike goes away after 90 days. I think YT made this rule to protect content creators like us, who may accidentally get a strike, but not regularily.
Especially in your case with a piece under public domain, older than 70 years you have chances to win. With what exact claim do they go against you? And who is the claimaint?
I remember a video someone said, for the claimaint itself it gets risky as well, because they have to prove their claim, too. If they can't, they have similar consequences as a strike, if not worse.
Just check, if Debussy is still alive, or if this piece falls under a different restriction of copyright duration. I read everywhere it is 70 years, but have no clue if this is limited to the US, only. What you definitely can claim for yourself is the right on the audio and video recording, because you played it by yourself.
What makes me wonder is why the claimant goes the 2nd step. Maybe a misclick? Maybe an attempt to scare you. Maybe it is a internet a****le who has various fake claimant accounts and don't case if one of those gets shut down. If enough accounts are running and enough content creators are scared, still some money steal from YT platform.
Disclaimer: I am not a laywer - just watched some videos. There are many videos, dealing this topic. And I remember the answer of Google-AI: telling it is worth to fight against, and about what to write as answer.
Wish you best of luck for a good decision. Maybe sleep over it, but not too long. AFAIK for your next step the time window is less than for the first step. A week or so. Or maybe 10 days.
