Google's John Mueller was asked by an SEO named Promothash Boruah when is it safe to delete tag and/or category pages on your web site. John responded that you can remove those pages with no real impact on your organic search traffic if those pages have "almost no traffic or no visibility of those pages."
This question came up in this past Friday's SEO hangout at the 12:30 mark where Promothash Boruah asked "I have a WordPress blog in which I have created some tags and categories. So what happens-- now I want to delete those tags and categories. These tags we created for some post, but these tags don't have any backlinks. And these categories also don't have any backlinks. And if I delete, what will happen? Google will down my rankings, or it will not."
John responded saying "So just because there are no external links to those pages doesn't mean that Google would not be able to find them. So that's kind of the first thing, because especially if you have posts that have tags and categories on them, then you have links from the posts to the tags and category pages." He then explained how to look to see if these tag and category pages have any visibility. John said "So what I would look at there instead of external links is how those pages are currently being shown. So in Search Console in the Performance report, look at the individual tag and category pages or the group of those pages. And then based on that, you can make a decision. Can I just delete these pages? Do I need to redirect them or not? If there's almost no traffic or no visibility of those pages, then cleaning that up is perfectly fine."
Here is the video embed with where this took place:
Often, SEOs do two things with tag and category pages. They either block Google from crawling/indexing them or they beef up the content on those pages so they rank well.
Forum discussion at Twitter.