I am curious to read other opinions. That said, simplicity has two axis:

The instruction set and overall CPU complexity, and the hardware it's running on.

In my view, playing with assembly is best done while also either playing with hardware, or really understanding and having access to said hardware at the "raw metal" level. The distinction being computation, and that's not too much different of an experience, and making things actually happen, which can be a widely different experience!

The simplicity, again in my view, of 8 bit type hardware is very helpful. It's not so large and or complex. People can understand it reasonably well, and with some degree of effort, make things happen.

Back when I was learning assembly language, it was on these kinds of systems. Sometimes the hardware played a lesser role, say a machine that has serial i/o for use with a terminal. Other times, the hardware was far more prominent, like with home computers of the time.

With the terminal, one would typically setup the hardware for serial comms, and once that was all done, basically stuff values, from a buffer, into the designated address or capture them, into a buffer, as they arrived.

On a machine with built in display, the display was typically memory mapped in some fashion. Writing to a specific address in RAM would cause a character to appear on the display screen.

It was common to have a memory map of the computer handy when writing assembly language programs. Put a number here, or set a bit there, and things would happen, like maybe click a speaker, or change the color of the screen, turn a motor on, or make a sound.

Have you looked at the 6502 hardware tutorial kits Ben Eater is doing?

https://eater.net/6502

(I mentioned up thread the idea of doing these with the 6809, which is tempting me badly right now...)

They are fantastic! Assembly language + hardware functionality = your program.

And this is the embedded view on things too. Having everything simple and small generally means being able to make good progress and learn a lot without it being too heavy of a lift. And depending on what your interests are, frankly you might be able to do more than you would expect.

Should your interests lie in this direction, a 6809 is an awful lot of fun to program. It's one of the best 8 bit experiences, IMHO. Be warned though! It can be an addiction, and once it starts... you may find yourself with a scope, spiffy new tools, and the need for some space to work, and, and, and! Mine has been life long, first triggered by writing a line of BASIC, pressing carriage return (enter key these days), and seeing a light bulb turn on. :D Soon after that, it was a speaker click, then a loop to make sounds, then one with parameters to make different sounds, and, and, and... There you go.

But, it's not necessary to fixate on this chip either. It's a beautiful CPU, no doubt. But others are also fun, and they have their advantages and there is a wide variety of hardware out there to program in and with.

If this kind of thing does not appeal in some way, then yes! By all means explore assembly language on something newer, bigger and faster.

Or, maybe it is, and or you don't know.

I would suggest emulation. We've got pretty great emulation for all the old machines.

Many of us, who enjoy these older CPUs, work with emulators, because we've got access to modern tools and such that make life a whole lot easier.

While there is something to be said for an authentic experience, to be frank? It's a lot of work, and kind of painful most of the time.

A good emulation smooths away a lot of pain points and "nicer" is a lot less important. Use an older CPU, newer one, freely. And you can save states and all the other good stuff that makes learning less painful and more fun and fruitful.

Right now I've got a little 6502 project in progress for an Apple //e type computer. It's being done in an emulator, and from time to time I move it all onto the real computer just to see it all go and interact with it for real, and or check it out at different clock speeds, or just show off.

But the majority of the time developing is on emulation. I can fire it up, make a little progress, or explore an idea, and then save everything for the next step later. In this way, it's easy to jump in for an hour, have some fun, and get back out again when that hour isn't costly in some way.

Probably more here than you wanted. Hope it helps.