Apple used to offer its customers the option of choosing between SSD and its Fusion Drive in terms of storage. This included 512GB and 1TB SSD options, but now it seems that neither of those are available. Instead, if you wanted a 21.5-inch iMac, you can only choose between a 256GB SSD, which is way too little storage, or the slower 1TB Fusion Drive.
This has led to speculation that the reason is because Apple is trying to phase out the computer, or at least try to sell out whatever remaining stock they have left ahead of the refresh. The rumors are claiming that the iMac refresh will not only introduce the use of the M1 chipset (or some variant of it), but it could also bring about a redesign of the desktop.
In fact, we have heard that the 21.5-inch model could be replaced by a 24-inch version. We can’t say for sure if these rumors are true. For all we know, it could be due to supply constraints which is why neither configuration is available, but either way we should have more details next month, hopefully.