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Google's John Mueller confirmed that using a rel=external, rel=noopener and/or a rel=noreferrer have no impact on your Google search rankings or SEO. John said on Reddit that there is "no effect" by adding those link attributes to your hyperlinks.
The question on Reddit was "What effect do rel='external' backlinks have on SEO?" John replied "No effect." He then added that "some sites use it to display the link a bit differently (usually class=external though). Also rel=noopener & rel=noreferrer don't have any SEO effect, they're usually done for security / privacy reasons. (Reading https://mathiasbynens.github.io/rel-noopener/ , apparently browsers now implicitly add rel=noopener automatically for links that open in new tabs - TIL)."
You can add a lot of markup to your site and links but Google does not support or use a lot of it. For any markup you want to use and you are interested in how Google uses it or not, it must be listed in the Google Developer documents for it to be supported.
Forum discussion at Reddit.
