Usernames have always been stored centrally in the US. That's similar to most global services for the IAM Layer. The only thing additional that gets stored in the US as part of UC is the metastore names. you can check this link ( https://azure.microsoft.com/en-gb/global-infrastructure/data-residency/)

Azure Databricks which stores identity data and certain tables names and object path information in the US.

Identity Data -> user & group names

Table names -> UC metastore names

Object path information -> Workspace ids