Syloti Keyboard is an app for writing the Syloti Nagri script, used for writing the Sylheti language, which is spoken by eleven million people in northeastern Bangladesh and India. The earliest known work using the script comes from 1549, and it was used continuously until the 20th century, when the language was discouraged in writing in favour of Bengali. Nowadays, it is experiencing a revitalization movement, with Sylhetis both in South Asia and in the diaspora looking to write Sylheti using its traditional script. Sylheti Keyboard is built in Swift for iOS 12.0 and above, and provides a keyboard for iOS users to be able to write Syloti Nagri on their devices. You can find the link for the app on the App Store here.
For non-iOS users
If you are interested in typing Syloti Nagri on Android, Gboard now has a Syloti Nagri keyboard available. In addition, there are a few standalone apps on Android as well, such as this one, but I have not used any of them myself. For Windows users, the Sylheti Translation And Research group have released an installer made in MSKLC for a Sylheti keyboard layout, along with their font Surma, which I highly recommend for anyone. For Ubuntu 20.04 users, I made a shell script here which will install the keyboard layout I personally use, based on InScript - no guarantees on it working on other versions of Ubuntu, or for other distros.