Archaeology is an odd one as it kind of did get updated but it also kind of didn’t.
It still uses the older style 1-950 progress bar instead of each expansion having it’s own bar, with some zones being unlocked as you progress.
Back in Warlords of Draenor to try and make leveling Archaeology a bit easier, you could do from 1-700 just in Draenor, instead of having to go through Classic, Outland, Wrath of the Lich King, Cataclysm and then Mists of Pandaria zones to skill up to the new zones from the new expansion.
The same applies in Legion and Battle for Azeroth, you can level from 1-max (currently 950 in Battle for Azeroth) just in these expansions zones.
The reason you can’t see digsites in Mists of Pandaria is because those still use the older system, you need to have at least 525 to see those.
So you’ve got a couple options depending on how you want to level and what it is you are after, and if you’ve got flying unlocked or not in Draenor and Legion 
- You could do it all in Draenor, Legion or Battle for Azeroth zones using the newer 1-max style leveling
- Or you could go through the older style of leveling, Classic zones, then Outland, Wrath of the Lich King, Cataclysm and then Mists of Pandaria zones as you skill up.
There’s a couple of guides on Wowhead, some of them up to date and some of them a little older but the majority of the info even in the Legion and Draenor versions is still valid: