It appears requestheader was intended to be used about a specific configuration option in Apache. But some questions have used requestheader where they should have been using http-headers. I saw only 10 questions in total with this tag.
I have created a tag wiki explaining that this tag is for the Apache
RequestHeader configuration directive. I have removed requestheader from three questions and replaced it with http-headers where appropriate. There are six questions left with the requestheader tag, which all appear to actually be about Apache configuration.
It seems like host-headers is covering one specific HTTP header. I have created a tag wiki explaining this. It looks like the majority of the 26 questions with this tag are using it consistently about the HTTP Host header. It might be useful to rename the tag as http-host-header. I have removed the tag from a single question about SMTP.
header and headers seem too generic to be useful. On a quick look it seems like maybe 40-50% of them should be retagged as http-headers while there is also maybe 30-40% about mail headers, and a few which are about other kinds of headers.