Over the last couple of days, Apple has removed the 512 GB and 1 TB SSD options from the 4K iMac configurator. Now, all 21.5-inch iMacs, including the non-Retina entry model come with a 256 GB SSD as standard, and can only be upgraded to a 1 TB Fusion Drive from the factory.
Apple has let the smaller iMac stagnate for a while now, as it didn’t see an update when the 5K models were updated last year. No one should be buying an iMac right now, but especially a 21.5-inch model.