Ok, let's clear things up, Kalafina is not disbanding, but only one member will leave the group. One of my friends who is the member of Harmony (Kalafina Fan Club) is confirming this news, so we decided to go with this news, and it's official as well.
Quote:
As many of you may already know, there are rumors about Kalafina's disbandment due to Yuki Kajiura parting ways with her and Kalafina's shared agency.
No official statement has been made by Kalafina's staff. However, the Harmony Fanclub members have received a message concerning Kalafina's future. It states that although Kalafina will not disband, one member will leave the group and the remaining members will continue as a duo.
Currently, Kalafina are still scheduled to appear in events through to the end of March. We hope this post will help to clear things up. 
The Harmony Fanclub e-mail contained the following message:
"To Harmony Fanclub members,
Today, news concerning Kalafina has surfaced from one media outlet, so we would like to inform the fan club members of the following.
One member of Kalafina has expressed intent of leaving the group, and upon receipt of said request, we have decided to accept it. The two remaining members will continue activities with our agency.
While giving utmost respect and importance to Yuki Kajiura's works through activities intended to express passion for said music, we would like to continue with our plans that all of the fans will enjoy and will be happy about.
We shall announce them soon.
We would like to ask for your continued support.
March 13, 2018
- SpaceCraft Produce"
Again, we would like to clarify that Kalafina are not disbanding. Regarding the change in their member lineup, we would like to encourage everyone to continue supporting Kalafina in the future.
(FC Mail translation credits to −suteki)
Harmony FC Mail link:
https://kalafina-fc-harmony.jp/contents/162600
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Still a better news rather than just disbanding, yet Kalafina won't be the same anymore.
I'm guessing the one who is going to leave is either Keiko or Hikaru. Why?
The recent 10th anniversary pamphlet interview reveals Hikaru's thoughts for so long, she said that her voice is somehow out of place, and saying that the combination of Keiko and Wakana is the true sound of Kalafina.
Keiko's interview was rather simple, something like, "Wherever Kajiura is going, I will always follow her." So, yeah.