guys i think i figured it out

Ruby and Sapphire (2002/2003)
FireRed and LeafGreen (2004/2004) no frontier
Emerald (2004/2005) had frontier

Diamond and Pearl (2006/2007)
Platinum (2008/2009) had frontier
HeartGold and SoulSilver (2009/2010) had frontier

the reason why oras have no frontier is because it is came out before the third game.
so logically the frontier will be in Z.
masuda just did not want to admit it was not added because of time constraints.
frontier was in heartgold/soulsilver because of how easy it was to just copy paste it.