HTML5 says it "must not be used by authors", but it still describes it as a tag, and describes what an implementation must conform to. I guess it's open to interpretation what "valid" means. But it is an HTML tag described by the HTML standard (both HTML5[1] and the WhatWG HTML spec [2])

[1]: https://www.w3.org/TR/2011/WD-html5-20110405/obsolete.html#t...

[2]: https://html.spec.whatwg.org/multipage/rendering.html#the-ma...