I monitor the Regency romance bestseller list, and there are several 'not available' messages there. I can't tell precisely which books are missing or whether they're gone from the store, but I can tell you that the main content mill publisher had several books in that list which were Western romances miscategorised as Regencies. Those have all gone, and one book which is American Civil War era is now showing as Regency. So my guess is it's something to do with a cleanup of categories, and Amazon broke something along the way.
ETA: And having said all that, the Western books are all back this morning, and the Civil War book has gone, so... maybe Amazon flipped the switch back?