There is something very strange, at least for me. Around august 2017 we moved our forum from vbulletin to xenforo 1.5. We then used a redirect script to redirect the /showthread url's from vbulletin to the new xenforo url's /threads. This worked fine and we did it to avoid that all the /showthread url's in google receives a 404 error. In januari we upgraded xf 1.5 to xf 2.0. However we did not implement the redirect script again, because almost all /showthread url's are not anymore in the google search results. And now the strange thing. Once we have made the update I see in GSC a large number of 404 errors. Most of them are the /showthread url's. For me it is a mysterie where google spidered/crawled these old vbulletin url's. In google there are only 5 left. In the sitemaps are 0 of the /showthread url's and in the database are 0 of these url's. I hope that someone have an answer for this 'problem'.