Very interesting. I found an issue with my site that I couldn't reproduce here in XenForo.com so I haven't made any suggestion about it.
My community has always made their own rules about several isues and one of them is "one thread per discussion topic". For example the current COVID19 thread started back in february and is 295 pages long so far. However, I detected a problem with my search results: Google was not indexing the latest pages of that thread anymore. I started investigating and the problem is Google indexed every page of that thread with a date of February 2020 even though there are new pages every day with current content.
Apparently Google was taking the thread starting date that appears below the thread title as the date that every page was written, affecting the number of pages indexed by Google because they thought the content was very old:
I decided to change that date to the date of the first post of that page and Google noticed and started indexing every page. Now, I haven't noticed a substantial increment in my Google traffic, but it's been a change that's been live for only about three weeks.
Again, this doesn't seem to affect XenForo.com but it was affecting my own site with an almost default style.
How to check if this affects your site? Go to a multi page thread spanning several weeks/months and type
site: before the thread URL and search that in Google. For example to test it with this thread's url you need to type this in Google:
site:xenforo.com/community/threads/sitemap-for-2nd-and-next-pages-too.93236/
If every result is from the same date that the thread was created, then you are affected by this problem. On the other hand, if every result's date is from the first post of that page, then you are not affected.
I didn't know about the sitemap not including every page of a thread but I think it could help Google to index my site faster and plan to do it soon. I will release an experimental addon for anyone who wishes to test this on their own site. I am not a SEO expert but it might help you in the long run.