The last Google archive before the last crash was on 2nd April, 23:49.

The last non-BS thread on that was 171175 (Lyr Req: Nick Dow Song) post #10 02 Apr 22 - 07:42 PM. (There may be later ones, which might be in caches available from other search sites.)

Thread 171175 seems to have been lost and the number reused.
https://mudcat.org/thread.cfm?threadid=171175
So I deduce it was started after 17 March (the date SRS said that the system had been been recovered to). A thread started before then should still exist but may have lost posts from the end.

Reusing the thread number seems to cause Google to remove cached versions of the original thread. (The mechanism is unclear; after all that thread was cached with a different '&messages=10' URL, but the upshot is that Google decides the cached version is stale and removes it.)

To find out how many, and which, posts have been lost you need the post numbers. These are not in the index page, they're in the thread page, and those are often not archived at the same time. After the restore the post numbers will also have been reused, I assume, but that matters less.

A few days ago I looked at the latest Google archive and estimated that 200 (BS and non-BS) posts had been added in the previous 24 hours, and that 24 threads had been added in the past 4 days.

If you wanted to create a window of opportunity, after a system restore, to search archives and caches, then maybe restore the system read-only for a couple of days. Or restrict posting to BS - which would allow you to explain what's happening.

But having a log of posts, as I suggested, would be better.