I had a secondary domain controller installed in a SBS 2008 environment. When migrating from SBS 2008 to Windows 2016 Standard, should I remove the secondary domain controller and is this documented on how to remove it?
You can keep the second DC in the scope of your migration. In one of the first steps of the kit I explain how to migrate FRS to DFSR and that needs to be performed only on the SBS 2008 as other DC's will follow automatically.
If you decide that you want to remove the second DC before you start the migration the steps are easy: