The Psions could do execute-in-place, including for third party software on ROM SSD - i.e. they did not have any fundamental distinction between working memory and disk storage.
Yes, NOR flash was common in these kind of devices and phones until maybe 2005 or so. So you could run code without loading it to RAM first. However, I believe RAM was still needed for stacks and heaps. Writing them to NOR flash sounds too slow.
No, I really do mean that they used the disks as ROM. No writing to it. This worked because you can just put the stack and heap at a difference position in the memory map from the executable code.