Barrapunto

No han reintroducido nada, los frames nunca han desaparecido. De XHTML 1.1:

This document type is essentially a reformulation of XHTML 1.0 Strict using XHTML Modules. This means that many facilities available in other XHTML Family document types (e.g., XHTML Frames) are not available in this document type. These other facilities are available through modules defined in Modularization of XHTML, and document authors are free to define document types based upon XHTML 1.1 that use these facilities (see [XHTMLMOD [w3.org]] for information on creating new document types).

Es decir, puedes usar frames en XHTML 1.1, pero tienes que usar el módulo (DTD) adecuado.