How to identify database users and their owned schemas before dropping server logins?
The query below will return the schema owner information.  You can change the JOIN to sys.database_principals to a LEFT JOIN to also return schema owners that are not logins, such as database roles and users without a login.

SELECT
      sp.name AS OwningLogin
    , dp.name AS OwningUser
    , s.name AS SchemaName
FROM sys.schemas AS s
JOIN sys.database_principals AS dp ON
    dp.principal_id = s.principal_id
JOIN sys.server_principals AS sp ON
    sp.sid = dp.sid;