According to the User management REST API changes in JIRA Cloud documentation the "Change user password" endpoint was removed because these APIs create, modify or delete user attributes and passwords, which are no longer managed by the JIRA tenant with the rollout of Atlassian account and SSO.
At the end of that documentation it says that "Atlassian accounts cannot be managed via public APIs today. We are currently evaluating how best to expose these capabilities in a secure manner that best meets our customers' use cases."
This request is to track the re-implementation of such endpoint.
This removal breaks some functionalities of some services like Okta.