This one got us too when we first upgraded. I knew I had uploaded showcat.php and was getting 404 errors. After looking at the upgrade code, it looks like one of the older upgrades deleted showcat.php. So for those of us who had to run the older upgrades to get fields, even though we started with 2.9x, it got deleted.
Might be good to make a sticky post for other folks so that if they started with 2.9x and had to rerun an older upgrade, to make sure to reupload showcat.