Can confirm that Threads of Fate is still available in the game. I just managed to do it using a Level 70 Druid. The key is to NOT USE CHROMIE TIME to access Shadowlands content. You cannot have done any SL content prior to this. Go to Darion Mograine in front of Grommash Hold or Stormwind Keep, and accept the starting quest from him. Then talk to Whitemane and select the Skip Option. This takes you straight to Oribos. As soon as you get to Oribos, Roh-Tahl will freeze you and give you the option to either do Threads of Fate OR the Storyline. If you want to just pick a covenant, go with Threads of Fate. Otherwise, pick Storyline.
But, confirmed, this option is still available in the game. Just so long as you have 1 lvl 70 toon that completed the original campaign.