It stays free to you to use or don't use this tag. It has no defined influence on SEO, and, in general, on any kind of web semantic. Even if the element was redefined in HTML5 after deprecating it in HTML4, HTML5 reminds developers, that there are almost always more appropriate elements than the
Beside of this all, using of
<u> is highly deceptive in terms of UX, because all human experience with the web browsers and HTML pages shouts: underline equals link! Even if it isn't blue - it could be just already visited. So if one tries to click on an
<u>word</u> and nothing happens - guess, what would one think about you as web designer? Well at this point we could say, yes: the
<u>-tag negatively influences a page in terms of SEO, because it fools a user, who doesn't want to be fooled and leaves the page.