Is Microsoft.SharePoint.Taxonomy.dll available with SharePoint 2010 Foundation?
No, because it falls into ECM bucket of SharePoint 2010 server. The best way to answer similar question is to look into SharePoint Server 2010 SDK. If the assembly is documented under Library reference of SharePoint Foundation, it is available for it and vice versa.
yes Microsoft.SharePoint.Taxonomy.dll available in machine where SharePoint is installed(GAC). this dll is provided by Microsoft when installation.