Didn't want to editorialize the Video title, but it's a bit of acknowledged clickbait. It's still a clever protected mode, implemented in simple external hardware that leverages Z80 features. Well worth a watch, if you're into that kinda thing.
Sounds like he is a good EE but was a bit frustrated with the growth of his channel numbers. Think his audience will give him a freebee this one time but do it again and they will nope out.
It would be possible to add more hardware to detect these instructions, so it could then trap and emulate them.
Zilog actually designed their peripheral chips to handle the RETI (return from interrupt) opcode specially. On the Z80 itself it does the same thing as a normal RET, but other chips can detect it on the bus and treat it as the signal that their interrupt handler is finished.
I also recall reading about some hobby project to add a PC-relative addressing mode to the 8080 or Z80. A redundant opcode like "MOV A,A" would be used as a prefix, which caused the external hardware to add the program counter to the immediate operand of the following instruction. Can't find it right now.
I guess it’s a short step to looking up memory addresses against a bitmap for validation. I couldn’t tell from the video if this is what he did. Cutting the addresses into pages would let you save or add more levels and I guess you could implement a TLB similarly.
6502 doesn't have a pin for "IO space" but you just pick your MMIO range e.g. 0xCnnn on the Apple ][ and a NAND gate on the 2 MSBs and a 3-input OR/NOR on the output and the next two bits gives you effectively the same signal. (or an OR gate on bits 12&13 plus a 3-input AND/NAND on bits 14&15 and the output).
It was common to use 6522 VIA 8 bit output ports to bank switch one or two 4k ranges to allow each such 4k range to access 256*4k = 1 MB of RAM. You could use a 16x8 bit SRAM to do the same thing for the whole address space.
It’s like the tweet- and video-ification of online discourse have destroyed peoples’ reading comprehension. (On reflection, though, the average person was never good at it. At least it’s mostly confined to annoyances on the Internet instead of, like, political elections or mob violence, right? Right??)