I predict that the last episode of Chapter 4 will end on a cliffhanger with Opaline about to tell the Mane Five what happened with Twilight and Equestria’s magic.
We will learn about that if it happens; but… in the book “Sunny Starscout’s book of adventure” Opaline made a very strange remark about Twilight and what happened to Equestria. “ - Her message glitched out, just like here Empire.” Such a vague description cannot come from someone who was supposed to be responsible for Equestria’s collapse and the disappearance of magic. She had admitted that she lost to Twilight, lost her magic and was banished and even forced to stay in the dark castle for several centuries. Those of us who know Twilight know she wouldn’t have let the damage left by Opaline remain if she had won.
The technology in the G5 made it clear that there was a world community when Equestria collapsed, and as a pony, Opaline would not call her own homeland an “empire”. It was too big for the ponies to just split up for this great society to collapse. We don’t know yet if the IDW covers from the Discord Arc are canon or AU - but if it’s the former, it means that Opaline can’t be responsible for Equestria’s collapse by herself. In fact, when Hologram-Twilight made the warning - the fear wasn’t for Opaline herself. The fear was that the fire alicorn will “open” something. What is it? And from what has been seen of Opaline’s own rhetoric and remarks, she seemed to be in ignorance of what Twilight fears.
I don’t think we will learn the whole truth so easily. But there are many indications that it was Twilight who sealed the magic.