I’m not going to lie here. I’m more than a little excited with the artifact transmog news going around. The spec limitation is gone.
If you’re a BM hunter that wants to use Thas’dorah, Legacy of the Windrunners (MM artifact) as your weapon mog, you’ll be able to in the pre-patch (9.0). If you’re a shaman that loves the Doomhammer (or even its off-hands), but doesn’t play enhancement, you’ll be able to use them.
I logged into the 9.0 PTR and tested the limitations. While I didn’t get screenshots of the hunter situation, I can confirm that BM hunters can chose Thas’dorah, Legacy of the Windrunners in your “Bows” section of the transmog interface. However, you cannot find it in the “Legion Artifact” section unless you’re in MM spec. While the “Legion Artifact” section still allows you to use artifacts of your current spec regardless of weapon type equipped, the actual weapon types (Bows, Polearms, Swords, etc.) follow the same rules we’re all used to. Every artifact main-hand and off-hand item has been added to its proper category (as well as still being in the Legion Artifacts section).
To really put this to the test, I hopped onto my shaman (restoration) that has a staff equipped. In the transmog interface, the only options I could find are the 2-hand Axes, 2-hand Maces, Staves, and Legion Artifacts. In order to use a 1-hand Mace and Shield transmog on top of a staff, I had to choose Legion Artifact and pick an appearance of Sharas’dal, Scepter of Tides (restoration artfiact). You cannot use the elemental fist/shield artifact combo nor the enhancement dual-mace artifact combo on top of a 2-hand weapon.
When I equipped a fist weapon and a shield, the main-hand options changed to 1-hand Axes, 1-hand Maces, Fist Weapons, and Legion Artifacts. The off-hand option was only Shields.
If you select Legion Artifact, the only options available are the different tints of Sharas’dal, Scepter of Tides. The elemental fist/shield artifact combo and the enhancement dual-mace artifact combo were not available.
If you select 1-hand Maces, you can select enhancement’s Doomhammer or Fury of the Stonemother (Doomhammer’s magma off-hand mace). If you select Fist Weapon, you case select elemental’s Fist of Ra-den. For the shield, The Highkeeper’s Ward (Fist of Ra-den’s shield) is an option. Meaning, if you equip the correct weapons/off-hands, you can use the enhancement and/or elemental artifacts as restoration.
This combination of magma mace and firey shield is something that I’ve wanted to be able to use on my Ele/Resto shaman since I learned of them during WoD. The happiness is very real and immense. No transmog is needed to show the smile I’m sporting.
Bodhi Rana 
It’s a good compromise – I do sort of wish that you could transmog Artifacts across regardless of weapon type, but it is a great move either way. As long as clothie main hand weapons aren’t wands in Shadowlands, this will be great!