The tells that mark a site as dated
Fixed-width containers built for a 1024px monitor, so the page sits in a narrow column on a modern display. Type set small and dense, without the generous line height that current layouts use. Heavy drop shadows, bevels and gradients applied to buttons and cards. Sidebars carrying navigation that has since moved to the header. And a separate mobile treatment that was added later and behaves like an afterthought.
None of these are content problems, which is why rewriting your copy never makes an old site feel new.