i agree about the registers and rst instructions
i don't see how octal could be obsolete; computers still use binary, and octal is just as useful for doing binary arithmetic as it ever was. as you point out, it's considerably more useful for instruction sets with 3-bit fields. moreover, the great advantage of hexadecimal was that bcd was easy to read, but that stopped mattering around the time the 8008 was introduced
i'm interested to hear if your experience differs, but i think it's a lot easier to do mental arithmetic in octal than in hex; it's a lot easier to remember 5 + 6 = 13, or figure it out if you forget, than d + c = 19, and both the addition and multiplication tables are one fourth the size
as far as i can tell, octal just went out of style. if anything, hexadecimal became obsolete, because its killer advantage went away, while its drawbacks remained
arm leaned into the hexadecimal wave and designed their instruction set to be easily readable in hex. we probably don't have to worry that a possible wide adoption of risc-v will initiate a transition to base 32, though; its instruction formats, while much simpler than thumb, resists any such easy numerical readability
btw, i don't remember if i ever told you this, but thank you for xfractint