It’s okay to have sponsored content on your website as long as you use nofollow links in the sponsored content. Google will see the content as a part of your website then. That’s what Google’s webmaster team said on Twitter.
Hi again, Dan! That's correct. If you don't mind that this content is seen as a part of your website, then just using rel=nofollow would be suitable. If you don't want it seen as a part of your site in search, using noindex would be better than robots.txt.— Google Webmasters (@googlewmc) August 26, 2019
Paid links should always contain the nofollow attribute
This statement is not new. Google’s Quality Guidelines say that paid links (for example links in sponsored content) should use the nofollow attribute:
“Additionally, creating links that weren’t editorially placed or vouched for by the site’s owner on a page, otherwise known as unnatural links, can be considered a violation of our guidelines. Here are a few common examples of unnatural links that may violate our guidelines: […] Advertorials or native advertising where payment is received for articles that include links that pass PageRank.”
If there is any kind of compensation for a link, it is better to use the rel=nofollow attribute in the link to avoid Google penalties.
Sponsored content should not exceed your own content
According to Google’s content policies, sponsored content should not exceed your own content:
“Advertising and other paid promotional material on your pages should not exceed your content. We do not allow content that conceals or misrepresents sponsored content as independent, editorial content.
“Sponsorship, including, but not limited to, ownership or affiliate interest, payment, or material support, should be clearly disclosed to readers. The subject of sponsored content should not focus on the sponsor without clear disclosure.”
If you want to avoid that Google sees the sponsored content as a part of your website, use the robots noindex tag on the page with the sponsored content.
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Among many other things, the website audit tool checks if your web pages contain noindex tags, nofollow tags, broken links, and much more: