If you google the name of any popular song, among the top suggestions will be " chords" or "tabs" or "piano arrangement", etc. Many of these hits will take you to forum sites where users post (what they believe to be) how to play the song. For songs with lyrics, the words of the lyrics are also often included. Depending on the song, site, and post you're looking at, the level of detail varies quite a bit, anywhere from a plain text file list of chords to full musical notation. One may also often find simplified arrangements, sometimes deliberate, sometimes because the poster only knows the simplified version and doesn't realize they're missing something.
Are such posts allowed under copyright law (US to be specific)? In order to make the question concrete, please focus on the case where it is only the literal, factual transcription of a recording. To me it is non-obvious. If I buy a book of sheet music and copy it, that seems a clear copyright violation. On the other hand, facts are not copyrightable, and it is a fact that the chord progression goes "C-G-Am" or whatever. Bonus points if you can also address transcriptions of the lyrics. I believe alternative arrangements (simplification or key transposition) would be called derivative works, but would be happy to be corrected on this as well.