GitHub Enterprise and GitLab support impersonation tokens (User Administration | GitHub Developer Guide), which allow a third-party application to impersonate a user when making API requests those services. This is useful for applications like Sourcegraph that fetch repository data and would like to enforce Bitbucket Server ACLs within their own UI.
Reading through the Bitbucket Server documentation, the closest thing comparable seems to be OAuth with impersonation (OAuth security for application links | Application Links Data Center and Server 7.1 | Atlassian Documentation), but that is only supported for first-party Atlassian applications. Does Bitbucket Server support impersonation tokens or will it at some point in the future?