For example, I've just now realized that,
When searching questions that contain the word "palindrome" but do not contain the appropriate tag
palindrome -[palindrome] , we get that there are over $600$ results.
I went on to tag around 30 such questions (increasing the total number of tagged questions from 70 to 100), but then I stopped mainly for two reasons. First, all of those questions will get bumped up to the front page and prevent new questions from showing in the period while the tags are being edited. Secondly, 600 questions is not a small number and will take some time.
I must admit that this is my fault because a few years ago, I was passionate about the properties of palindromes and was the one to create the tag (by simply adding it to new questions I was posting), but, I did not consider that a lot of old questions that have been asked in the past and are related to this topic, will now be missing out on this tag.
Should we, and how, tag these questions?
Is it possible for someone (a moderator or admin?) to retroactively add the tag to all these questions?
Or should every such question be reviewed individually and tagged appropriately? (Is it okay for me to continue tagging every question individually?)
Should we NOT tag these questions?
Or is there a third option that should take action?
I.e. not all questions that involve prime numbers have the tag
[prime-numbers]. Therefore, should all these questions that are missing the tag
[palindrome] just be left alone?