Zones will generally fall into one of 4 categories:
- Places you can never fly
- Places that require an achievement to fly
- Places that just require having flying trained to fly
- Dragonriding
For the first one, it’s just something you’ll learn as you go. It’s fairly common for end game zones to not have flying, and there are some older zones that weren’t build for flying like the Blood Elf and Draenai starting zones.
The second one applied to Battle for Azeroth and Shadowlands. The important thing to note is that these achievements are account wide. So, once you’ve unlocked them any character you make will be able to fly in the zones as soon as they are high enough level. So you only have to do it once.
Some of the stuff will be a lot easier to do if you wait for level 70. So, you’ll make progress while you level (assuming you level in BfA or Shadowlands) but once you hit 60 move on to Dragonflight and come back at 70 to work on it as you have time. (It can be a nice side project to work on when you need a change of pace from Dragonflight.)
Battle for Azeroth has a fairly involved set of achievements you need. These will include things like doing most of the quests, grinding out the various reps in the expansion to revered, and exploring zones. There are two “meta” achievements you need. Battle for Azeroth Pathfinder Part 1 and Battle for Azeroth Pathfinder Part 2. (Meta achievements are achievements that are made up of other achievements.) You can find these in the exploration section of the achievements panel.
Shadowlands has two different achievements for different areas. For the main zones, the requirements are just questing related. You need to do the leveling campaign, your Order Hall campaign (which unlocks after you do the leveling campaign) and the first part of the Korthia campaign. (Basically, just keep following the quests.)
The final end game zone, Zereth Mortis has it’s own achievement for flying there. It’s a lot more like the Battle for Azeroth one where there are a number of tasks to complete. The achievment here is called “Unlocking the Secrets.”
The third catagory is pretty self explanatory, and for the fourth, Dragonriding is a feature of the new expansion (so not something you have to worry about until 60.) You can’t currently use traditional flying in the Dragon Isles, but you can use Dragonriding. (It uses special mounts and is a more active process than traditional flying.) You get introduced to it pretty early on once you start Dragonflight. They will likely add in a way to use traditional flying there at some point in a similar manner to BfA or Shadowlands, but we won’t know until they announce something.