The stock maps so we can mod them? IE: Breaker, Fragmentation Highpower ETC TC
Peace!: 
As per the patch notes here:
https://support.halowaypoint.com/hc/en-us/articles/10581554050324#h_01GH0PWWKA28ABEZ6D1JTN3ZPC
The Forge Beta will support canvas maps and not developer-made maps (such as Live Fire, Recharge, Fragmentation, etc.). Support for developer-made maps will be implemented in a future update.
Well That Really Sucks Big One! 
But yet we were able to edit stock maps from previous halo games! Thanks for the heads up bud 
Peace!
I don’t get it dev maps doesn’t support, but you’re able to forge on them. I launched streets in forge for example.