Similar to The War Within the ability to use Steady Flight in the new zones will require exploring all zones and completing the main story questchain.
Just like War Within I don’t think this is a good decision. As I’ve said before I do not use steady flight ever since it is just objectively worse than skyriding and since the ability to toggle between the two it is clearly the more accessible option. An argument to triviality could be made that it is only a few hours of questing+flying to get it done, but that goes both ways. Why have it be locked behind an achievement at all?
It feels odd for this to be a “Reward” for the Pathfinder achievement. It is something that should be baseline. If a reward is needed it could be a housing decor item or perhaps a mount like the Soaring Skyterror in WoD when pathfinder was first introduced.
It definitely is weird that a “worse flying” is the version we unlock. Might be interesting at minimum if old flying was much faster and could actually compete with dragonriding but that isn’t the case.
As far as what I prefer I do like leveling the first time on foot and then unlocking flying account wide. This has been my favorite way to level up but when it launches with flying and you’re competing for ressources/quests it doesn’t make sense to “nerf” yourself and not fly.