Yes it is true. Starting with the 2007 Mustang, Ford found it in their better judgment to do away with the pop out speaker grills unlike the 2005 through 2006 models.
ETA: I learned this the hard way by helping a friend replace his speakers in a 2007 GT.
yes...... your completely correct, 2005 to 2006 was designed properly for easy removal access of the rear deck speakers, then ford decided to get stupid in 2007 (their known to do that) and change design so that you have to almost literally remove half of your interior parts just to remove/replace rear deck speakers. You have to remove rear seats, remove rear side panels and then you can slide out the package tray, plus the fact the grille in the package tray seems to me has too much material blocking the speaker hampering performance.