The claim is debunked by The Quint fact-checking organization. Mani Ratnam (filmmaker) had signed the letter on Mob lynchings.
The story goes that the filmmaker Mani Ratnam denied to sign in the letter on mob lynching, The Quint concludes according to the inquiry with the Mani Ratnam's team and they had confirmed that he had signed the letter. Further, she said it is misinformation which is spreading on social media.
According to Times Now News article, the 49 celebrities' wrote an open letter by the social activists, filmmakers and artists like Aparna Sen, Mani Ratnam, Anurag Kashyap, Binayak Sen, Soumitro Chatterjee, Adoor Gopalkrishnan and so on were made a sign on the letter.