Hi @Amit Gupta, thanks for asking about the REST API's for independent (Agility) projects.
The correct API's to use are the ones documented under https://developer.atlassian.com/cloud/jira/software/rest/, however we are aware of some inconsistencies with the new independent projects.
We will be releasing more information and next steps soon, which I will share here as soon as it's available.
Hi @Amit Gupta
Just to follow up, more information on our support for REST API's on next-gen projects can be found here: https://developer.atlassian.com/blog/2018/09/jira-cloud-next-gen-projects-and-connect-apps/
Amit. the documentation located is here
Most of thing based on the Jira core things, but agile things you will find through provided link.