Please, could you provide me a URL to download R6.x Java SDK? I cannot find it in the Downloads section of the Support site, and as we are using CA SiteMinder 6 in some projects, we need it to create Policy Management applications.

Release: ETRTMA99000-6.0-TransactionMinder-Additional XML Agent CPU's
Component:

Please, note that R6.x release is currently EOL & EOS, since end of March 2013. You can find the announcement below: 
https://support.ca.com/us/product-content/status/support-life-cycle/revised-end-of-service-announcement-for-ca-siteminder-6.html 
For SDK Agent R6, there is the following announcement stating it is also EOL & EOS since December 2016. Extended support is also ended since December 2017: 
https://support.ca.com/us/product-content/status/announcement-documents/2016/ca-sso-r6-agent-sdk-end-of-support-announcement.html 

Then, the packages are not available at the Support site to be downloaded, and from Support we cannot provide them as well anymore. The best recommendation is to upgrade to a supported release where you could also get benefits from all new features along with support for all your components. You can find the current release lifecycle at the following location: 
https://support.ca.com/us/product-content/status/support-life-cycle/indexes/ca-single-sign-on-release-and-support-lifecycle-dates.html 

Also, you may want to contact CA Services team for guidance and assistance if needed for the upgrade process, or reach to your CA Account Manager for alternatives.