The Mill architecture does support IEEE Decimal, although only some members will have it in hardware; the rest will use specialize-time emulating substitution. Decimal8 and Decimal16 are test configurations (topping out at 8- and 16-byte decimal representations respectively) for markets that want it: mainframe, DB, and Big Data mostly. As test vehicles they won’t be products, but derivatives may be, when and if we decide to tackle those markets; don’t hold your breath.
Mini is a straw man to see how small we can get the specification before execution becomes completely impractical. It will never be a product.
Maxi is a straw man at the other end, to see how muscle-bound we can configure a Mill before it hits diminishing returns with the clock rate. Also not a product per se, although there may be products between Gold and Maxi.
The reason Maxi needs so big a scratchpad is because it has a huge vector size, taking whole cache lines as single operands (SIMD) and lots of them (MIMD). It won’t have that many more operands in scratch than say a Gold, but they will be much bigger on average (no one would use a Maxi for anything but vector-busting numeric work) so the scratch size, in bytes, must be bigger.