Comment by ndiddy
MIPS is a load-store architecture and has branch delay slots, both of which make it complicated to learn for beginners. If you want to learn an obsolete ISA with no commercial relevance because it's simpler, I would recommend 68000 or 6502 instead.
Those are the ones I was fed in the early 00s and I can confirm they work fine for pedagogical purposes. But if it were my choice today, I think I'd go with THUMB on legacy 32-bit ARM chips. (This can even theoretically be used practically today: GBA games can be emulated on 3DS and probably also Switch.)