On our Sitecore 9.0.x environments we have a file at Website\App_Config\Include\Sitecore.Upgrade82.config with the following contents.
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:set="http://www.sitecore.net/xmlconfig/set/">
<sitecore>
<services>
<register serviceType="Sitecore.Abstractions.BaseFactory, Sitecore.Kernel" implementationType="Sitecore.Upgrade82.Configuration.BackwardCompatibleFactory, Sitecore.Upgrade82" lifetime="Singleton"/>
</services>
</sitecore>
</configuration>
Is there any reason to keep this file after upgrading to Sitecore 9.x? If not, is there anything else related to this file that can also be removed?