Keywords are added metadata, and added metadata are stored in the catalog. The reinstall of your OS is not relevant because even if there were any remaining Excire files, then these could not add keywords anyway. I'm puzzled by your remark "The keyword is only added as soon I have exported the images from Lightroom Classic". Lightroom Classic does not have an option to add keywords during export. Or do you use a special export plugin (LR/Mogrify?) that can do that? The only other explanation is that they must have been in the catalog all along.
Yes, you are right. I know meanwhile what the issue is. I type in my keywords....e.g. NATURE....when I start typing, Lightroom proposes the full word, I type ENTER and NATURE is saved.....but this keyword seems to be derived from a Excire-keyword-list an adds Excire to the keywords. I can only see this when I switch from "Enter Keywords" to "Keywords & Containing Keywords". So, it seems, I have to clean up my keyword list to get rid of these Excire-tags.



Addition....here is the root of the problem.....
