Posts or whole threads are sometimes removed by the admins, but that seems to me to be infrequent. (One did happen recently.) If a whole thread is removed, they do not necessarily notify each person who may have posted in the thread.

In the past I found a way, using the History cache of my browser, to be able to go back and reproduce an old post even when it was gone from PC. Since then something seems to have changed in the browser, and I no longer know how to do that. You could not just go back into browser history and click on the old link, because that would connect you to the no-longer-existing place on PC, and that would wipe out information about what used to exist.

Now I have given DocJohn too much information...