I saw this on the site of 1 Peter 5. Take a look at the video he made. I tried it myself and saw the same thing.
He said someone told him "never underestimate the incompetence of the Vatican IT staff". I don't think it's innocent incompetence. Consider that before September (or whenever it was removed) that the links to the popes' writings had existed there for years. Only with a fair degree of deliberation can a link be changed. Notice that the link isn't merely broken as though the Pope Pius XI page was inadvertently deleted; that would have triggered an error message. It is rerouted back to Pope Francis' page; someone had to change that page link. Moreover, if it was some technical issue, don't you think that the pages pertaining to other popes might be likewise impacted? They aren't - just that of Pius XI.
Skojec has suggested reasons why Vatican progressives might find the writings of Pope Pius XI to be rather inconvenient to their goals of instilling de facto disobedience to Christ's moral teachings. I agree but also think there are additional causes. Recalling that Amoralis Lamentia has done much to instill "pastoral malfeasance" regarding divorce and remarriage, consider that with the other writings of Pope Pius XI encyclical Casti Connubii likewise disappeared. Another encyclical that's gone is Quadragesimo Anno, in which Pius highlighted socialism's incompatibility with Catholicism.
Some might be rather indignant that we'd suggest Vatican officials might be capable of such underhanded actions. Time for a memory-refresher. Remember "Book-Thief Baldiserri"? Read the link and you'll see how this website alteration might just be right up their alley.
Fortunately the internet is what it is, and there are other sites on which papal writings are available. One of the sites is papalencyclicals.net. Take a visit. It is still a scandal that on the Church's website, the writings of a pope could be removed.