Sounds to me like there are some redirects missing after migration.. So other sites linking to your site etc are now going to a 404 (Not found) or 403 (Forbidden) pages.. This is not optimal, as you will then be "less relevant" for those keywords from where the links are coming. (which in turn leads to lower rank in search engine results)
For VB3 there was some custom script needed to handle redirects to new "url format", not sure if those are correctly handled by XF when it is migrated from VB4 ?
If you have access to
Ahrefs, it might make sense to see if there are a lot of backlinks going to "broken pages" (403/404) on your domain
- I have access, if you PM your forum domain, I can export a list later today
Otherwise, I would install
this 404 addon which will find pages where visitors are trying to "come in" (from other sites etc) but are being blocked.. See if you can find some "pattern" over the broken pages.
- If there are many similar ones, I doubt it makes sense to redirect them one by one, but a "programatical solution" would be the way to go
I bought a forum in 2019 that had previously been VB3 -> XF1.5 migrated, and first thing I did was to find old backlinks that went to "broken pages" in Ahrefs, and I fixed the VB3-redirection script to also handle those missing cases.