FWIW, it's not deprecated. It has been removed from the language. HTML5 does not do deprecation. By your reasoning <plaintext>, <isindex> etc. should also be conforming which doesn't make much sense to me.
So you don't see any difference between documents using style attributes (allowed in HTML4), and documents using <isindex> (deprecated in HTML4) or <plaintext> (not present in HTML4)?
Best regards, Julian
