I heard from a guy at work that it's forbidden to add keywords to a keyword meta tag that do not exist in the page's content, and that hurts your SEO ranking, which sounded pretty odd to be honest.
Does anybody have any input regarding this?
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For Google, it is not necessary to use a meta keyword tag. It won’t help in search engine optimization. It is seen as a spam signal rather than a ranking signal. Adding keywords to a meta keyword tag could also give your competitors an idea of what keywords you are using. So don't waste time on them and focus on quality content.
Here is a video from Google webmasters
Here is a blog post from google webmasters blog https://webmasters.googleblog.com/2009/09/google-does-not-use-keywords-meta-tag.html
Google has changed lots of things about meta tag, now there is not necessary to add meta keywords tag, it is optional now. If you want to add you can add, Google will only content keywords on the content, not on the meta keywords, so it is Is not necessary for all the keywords in a meta keyword tag to be included in the page's content.