If so, that might be along the lines of saying that an implementation that supports a new architecture extension is not required to be able to run "old/legacy" M-mode boot software that is unaware of the possible existence of that extension, and is not required to then run all other extension-unaware system and user software as expected while the extension remains enabled.  Conversely it would be the M-mode boot software's responsibility to properly either disable a new extension, or initialize the extension's relevant architectural state so that other (well-behaved) extension-unaware system and user software can run as expected.

I think it's the kind of thing that belongs in the FAQ that Mark mentioned in his previous email.