The Void as a concept didn't exist until TBC. Even in TBC, the Void wasn't a cosmic force like it is today. It was thought to be a geographical location (like the Twisting Nether) or a some evil faction (like the Legion). A strong connection between the Void, Shadow Priests, and the Old Gods didn't come about until MoP/Cata, and it was still pretty weak or obscure. This is the earliest source I can find any information on that at least loosely connects Shadow Priests to the Void: https://wow.gamepedia.com/World_of_W...ume_I_Issue_IV. It mentions the Cult of Forgotten Shadow, but I don't know if it ever calls Shadow magic as "void magic" at any point.
It didn't come out of nowhere in Legion, but it definitely was barely developed at all before then.
If you want actual evidence, just look at the Void wiki page.
From 2007: https://wow.gamepedia.com/index.php?...d&oldid=769051
From 2012: https://wow.gamepedia.com/index.php?...&oldid=3042635
From 2013: https://wow.gamepedia.com/index.php?...&oldid=3128337
The Shadow magic page is similarly bereft of any information from before 2017: https://wow.gamepedia.com/index.php?...&oldid=3388055
In fact only after Chronicles was it ever actually updated to include references to the Void or the Old Gods: https://wow.gamepedia.com/index.php?...&oldid=4547580
Here's the article on the Old Gods from 2015, no connection to the Void or Shadow: https://wow.gamepedia.com/index.php?...&oldid=3548639