Interesting topic, I do understand about why some of them show up as invalid per another thread about this subject I found -
"An Invalid Thread is one that has either been:
-moved and the redirect has expired;
-merged and the redirect has expired; or
-deleted because it did not comply with policy
A closed thread is one that remains visible but can’t have new comments added. "
I found one thread just this morning that was completely deleted even though the thread was started at least 4 1/2 years ago. It had at least two posts from 2015, one in 2016, one in 2017 and then one this morning. I do understand why the one in 2017 would be deleted, but why not then? and why now and the whole thread?
Janey - Neat people never make the exciting discoveries I do.