Explaining on the itch page is good and all, but that doesn't replace explaining it within the game. A player shouldn't be expected to learn something from outside the game in order to know how to play it, that is not acceptable.
Along with that, this description of "remixing the suggested line" is actually STILL really vague and it doesn't get across well what that translates to in gameplay. What is "remixing" a line? That could mean a lot of things, and it's not an obvious conclusion for it to mean "make up notes that don't appear in the song", especially when the game trying to do that tends to sound bad unless you're already really good at making it sound good, and the game has already explicitly told you to play the notes that appear on the screen.
It's very strange they would remove that NPC, and it's really concerning to see them commit to doing that considering that while yes, having an NPC just describe textually what a mechanic is not good game design, just removing ALL explanations of the mechanic without replacing them is infinitely worse. I wish I could assume they will just put it back but I cannot imagine a reason for them to remove it to begin with.