A Microsoft tool designed to automate database migration to SQL Server from Access, DB2, MySQL, Oracle, and SAP ASE.
Older versions of SSMA for Oracle are available through the Microsoft Download Center archive. Search for "SQL Server Migration Assistant for Oracle" on https://www.microsoft.com/en-us/download and filter by date, or check the release history page in the official SSMA documentation on Microsoft Learn which lists prior releases with direct download links.
The common reason for needing an older version is exactly what was noted above — newer SSMA releases require newer Oracle client versions that won't connect to legacy databases like Oracle 10g or 11g. Using an older SSMA build that matches your Oracle client version is a legitimate workaround when upgrading the source Oracle environment isn't an option.