ARM or one of the smaller RISC-V flavor-sets (RISC-V is super-modular) would be a perfectly reasonable architecture to learn these days.  After the PDP-11 but before ARM I'd'a suggested 68000.  Definitely NOT x86 and its betentacled descendants.  Even so, you'd still want to treat it (if you're learning "how do computers work?") as if it were not superscalar, even though it obviously is.  Which I guess is pushing me into "please let me just pretend it's a PDP-11 and keep all the scary pipelining and speculative execution and all the things that are hard to reason about below the layer where I need to care" territory.