Robbie Collin has written a wonderful tribute for The Telegraph
Some reactions:
Studio Ponoc: On the passing of director Isao Takahata… With our condolences and sympathy to his family and friends... We were honored and humbled to know you and work with you, and are deeply grateful for all you so generously gave us and the world. Thank you, Takahata-san. Studio Ponoc
Yoshiaki Nishimura (Producer of Princess Kaguya): Takahata-san gave me the gift of the most bountiful experience of my life. Meeting him, sharing time, talking, laughing...even getting angry together. I most certainly will never forget the great pleasure of making a film with him and his boundless passion for films and peace. Takahata-san, thank you very, very much. Takahata-san, the world's finest animation director, will forever be my teacher.
Lee Unkrich (Toy Story 3, Coco): Really sad to hear about the passing of Isao Takahata, co-founder of Studio Ghibli. “Grave of the Fireflies” is an amazing, emotional film. And “My Neighbors the Yamadas” is incredibly charming. It also had a big influence on Michael Arndt’s script for “Little Miss Sunshine.”
Tomm Moore (The Secret of Kells, The Song of the Sea): Such sad news to wake up to - Takahatas work will Continue to inspire and influence so many of us - it was an honour for me to meet him when he was in LA for the oscar nomination of Princess Kaguya - a daily reference point of discussion for us on Wolfwalkers
Jorge R. Gutierrez (The Book of Life): No animated movie has made me cry more than this tender, gorgeous, profound, soulful and landmark film. A giant of our art form has left us. RIP maestro Isao Takahata
Peter Lord (Co-Founder of Aardman): Very sorry to hear about the death of this terrific director and delightful man.
Rian Johnson (Looper, The Last Jedi): Grave of the Fireflies (directed by Isao Takahata) is tied with Make Way For Tomorrow for the saddest film ever made.
LeSean Thomas (Cannon Busters, Children of Ether): Today we lost a giant in Isao Takahata; Among many things, catalyst in helping change the face of animation in Japan with his debut film, introducing us to a young talent who would later be a legend and the other half of founding Studio Ghibli. Rest easy Takahata-Sensei.