Do we still need to display the title on the header, if the title already can be seen on the sidebar like in the wireframe?
Not all users can 'See' the title.
Headings can help “chunk” and label groups of related content. And people who use assistive technologies such as screen readers can quickly and effectively navigate a page by its headings. When you make things accessible, they become easy to use for everyone.
Also, headings are required to meet the AA standard by WCAG SC# 2.4.6
Yes, I believe it's still very important to display the title.
- Title helps and aids the page's hierarchy
- Better SEO. source
- Responsive (in case you need to hide the navigation for mobile, or if it is a feature even on desktop)
One of the best examples would be this very website we're on, if you click on "Questions" for e.g. you will notice that there is a title on that page "All Questions".