That is documented here actually:
https://docs.discourse.org/#tag/Admin/operation/adminListUsers
but all
is not a valid “flag” to pass in.
You will need to paginate to get all of the users.
http://localhost:4200/admin/users/list/active.json?show_emails=false&page=1
http://localhost:4200/admin/users/list/active.json?show_emails=false&order=&asc=&page=2