Indeed, even with 256Kbit dram chips you need more than 32 of them before 20 bits becomes a limit. But 24 bits fit nicely in 3 bytes which works well both with register and pointers in memory, so why not? An application or hardware can always ignore address bits it doesn’t need.
I haven’t seen it, so can’t compare it. Let me check… ah, the datasheet is from 1982. At first glance it seems like MOS’s answer to Intel’s iAPX432 and given the time frame it is not hard to imagine why they abandoned it. At least it could also execute 6502 code. There have been more modern attempts to extend to 6502 to 32 bits but I think the Acorn people were right: you might as well just do a RISC.
There are also processors with TinyBASIC in ROM. A conventional processor with an internal Forth interpreter certainly looks like a Forth processor to the user.