We haven't been merged into Twitch, rather, we have replaced one authentication system (Curse Auth) with another (Twitch Auth) because Curse Auth no longer exists.
For full context: I created the Minecraft Forum (and Minecraft Wiki) independently in 2009, around the time that Minecraft first became available because Notch didn't want to run a forum. Eventually Minecraft became very, very popular and so a company called Curse approached us asking if they could acquire the forum (and wiki), so we sold the forum (and wiki) to Curse. After the acquisition the Minecraft Forum login system was replaced with the Curse Auth system, so to log in to the Minecraft Forum you had to use a Curse account and you could use the same account to log in to other websites, like the Minecraft Wiki. Then in 2016 Curse was acquired by Twitch, and the process of sunsetting Curse Auth began, which meant migrating all Curse websites (which includes this forum) away from Curse Auth and onto Twitch Auth.