Is it legal to take screenshot/photograph of a "disappearing message" and share it?

Yes. While taking a screenshot/photograph is not a 'crime' in itself, sharing it without consent can trigger severe legal consequences under the Bharatiya Nyaya Sanhita, 2023, and the Information Technology Act, 2000.

Under Section 66E of the IT Act, capturing or publishing private images without consent is illegal. Furthermore, if the message contains intimate content, Section 77 of the BNS (Voyeurism) applies, carrying a jail term of up to three years & fine. 

However, sharing a screenshot/photograph with authorities as evidence for harassment or a crime is legal under Section 63 of the Bhartiya Sakshya Adhiniyam, 2023.

Intent and content define the legality.