Clearing all this up:
The flamereaper artifact weapon appearance was made available to demon hunters who completed the havoc mage tower artifact challenge on the broken shore. The mage tower challenges were single player scenario challenges that existed for every class and spec. Completing the challenge would award you with the artifact appearance for that spec.
The purpose of the artifact challenges was to provide challenging and exclusive content for players during the legion expansion. The artifact challenges have been removed form the game with the start of BFA and the rewarded skins are no longer obtainable and will never be obtainable in the future.
However, if you did unlock the base appearance by completing the mage tower challenge, you can still unlock the secondary colours for that weapon specifically. As long as you have the base appearance for that spec.
tl;dr: The appearance is unfortunately no longer available and will never be available in the future.
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