well i found the gameinformer article that said what was cut and part of the improvised part is still there
here it is verbatim
"The now officially titled Guitar Hero: World Tour has lost one major feature that was discussed in our June cover story on the game (issue #182) The game's Jam Mode, which would have allowed players to freely improvise over chord changes and scales from the game's soundtrack has been removed. Thankfully, the game's more complex music creation studio mode will still be included in the game, allowing players to create their own original GH tracks."