- Legacy ID
- Install CO
- Install AiA: https://dev.withsix.com/projects/all-in-arma/wiki/All_in_Arma_Install_guide
- Delete AiA_Worlds.pbo in @AllInArma\Core\addons
- Load attached mission
- Start to game
- Fly westwards to the map border
Yes it is limited to A2/OA maps only.
Which is the vast majority of maps the community happens to like and are actually small enough not to force you to 10 FPS while online.
Also after all the work the community has done over the years it's absurd to just throw it away.
Backwards compatibility is one of the main issues modders have with this game because every 4-5 years we have to start over and trash everything we did before because the engine doesn't support them anymore.