Even today there are companies that still make accelerators for the 68000-based Amigas, but I've never seen any such accelerator with more than 256 MiB of RAM. If the 68030+ can address 4 GiB, then why don't we ever see that configuration? Is it a limitation in Workbench, or compatibility issues with older software, or something else?
Actually, it's 256 MB only for A4000/T CPU cards - it's 128 MB for A3000/T cards.
The reason for this is that Fat Gary asserts the _RAMSLOT signal for this memory window. That way, the CPU card knows when to activate the RAM controller and to run an internal cycle.
Overcoming this limit is (probably) possible but would require a complete address decoder and overriding the decoder on the mainboard.
3.5 Other Local Bus Signals
Local Slot Memory Decode (/RAMSLOT)
This is an address based chip select for the region of memory allocated to the local bus slot, $08000000-$0fffffff.
AllocEntry() sets bit 31 of the address to signal failure, so cannot indicate success for addresses beyond the 2GiB boundary. This limits AmigaOS to slightly less than 2GiB of general-purpose RAM due to ROM and I/O also sharing the space. Zorro III cards can (and do) use address space beyond 2GiB, even for RAM such as a frame buffer, so long as it's not linked into the system memory pool.
I'm not aware of any reason for a 256MiB limit, and so the lack of availability of larger boards is probably just because there's no commercial incentive to make them.
It probably has more to do with practical use and physical constraints: are there any uses that would warrant adding more than 256MB of RAM?
Emulators support up to 1.5GB of RAM, which indicates that the address space is available and that (at least some) software copes with it. The 68030 CPU has 32 physical address lines so there's certainly no limitation on that side (up to 4GB!).