No, the u tag is valid. It was briefly dropped/deprecated before html5 but saner minds prevailed and it was kept.
If you prefer to use a different tag, then create and use a clip.
Quote:
Modern Semantic MeaningIn HTML5, the <u> tag is no longer just for visual styling. It is now formally known as the Unarticulated Annotation element. It should be used to mark text that:
Has a non-textual annotation: Common use cases include indicating misspelled words or proper name marks in Chinese text.
Is stylistically different: It offsets text from its surrounding content without conveying extra emphasis or importance.
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It works in both epub2 (but deprecated) and epub3.
Perhaps you are using an older version of epubcheck? Or maybe epubcheck is holding to the XHtml 1.1 spec too literally.
Either way you can use a span with a class the does underlining and create your own clip if you want. Not sure how much longer support for epub2 will remain. Even the original spec is hard to find anymore!