A few years back in about 2010 I ran the W3C.org validity checker tool on a Drupal 6 website that is no longer exist and I clearly remember getting a message like usage of <u> tags isn't valid or a similar phrase about <u> tags.Did someone here ever heard of any recommendation not to use these tags? I find no reason whatsoever as to why this would be marked invalid.
Is using the  tag invalid or bad for SEO?
seo;html;validation;w3c validation
The reason it was marked invalid is because it was deprecated, i.e. at the time you tested it was no longer part of the current HTML spec. It's now been reintroduced to the HTML5 spec, albeit with a different use. Helpful discussion here.While there is a relationship, technical validity and desirability or efficacy for the purposes of SEO are not necessarily equivalent. You'll see plenty of technically invalid code performing perfectly well for SEO, perfectly valid code making no difference to SEO, and so on.In short, provided you're using the <u> element as specified, it may or may not have SEO advantages but certainly won't be an SEO problem. 