Twitch flags a lot of music in the public domain as copyrighted incorrectly. I believe this is primarily, or at least partially, because of the following reason:

When there's a song in public domain several recordings of that song may exist, and some of those recordings may be proprietary and copyrighted.

The automated system misidentifies a song in the public domain as copyrighted due to there existing a similar recording that's proprietary.

For example Petrucci Music Library (imlsp.org) contains classical sheet music and recordings of that music. Most of the recordings on the website are in the public domain - not all - and most of them get flagged as copyrighted by twitch's system.

For example a composition by Beethoven is obviously in the public domain, but there are several performances of all of his compositions, and some of those recordings are copyrighted. Some recordings are only copyrighted in the US or EU, but not both. Many are in public domain, but are still flagged due to similarity with copyrighted recordings of the same compositions.

The short version: A lot of classical music online is in the public domain, but the system twitch uses for recognizing copyrighted songs makes the same error with almost all classical compositions. Hence it would be a good idea to contact the copyright check provider, and ask them to fix their system so that VODs containing classical music don't get muted due to faulty systems. Mentioning the imlsp.org reference might be a good way for them to adjust their system for classical music as well.

Obviously, users going through a lengthy appeal process to unmute their VODs is an ineffective method, when there's such a large categorical problem with classical music. And people don't like being misjudged as doing something wrong, when they're in fact doing things properly.