Is that actually true? I'm not convinced IBM and other PC manufacturers made such a mistake, but I'm happy to be proven wrong. It sounds a lot like it could be some sort of "common wisdom", because some ports presumably were dangerous to hotplug, so laypeople were just told to generally be cautious.
FWIW, I never followed that advice for anything as far as I can remember (save for really obvious stuff like ISA or PCI cards, or in professional settings where I was following some procedure), and nothing ever blew up. Could have been lucky.
(Super-IO was very late, by the way, it was 8042s and derivative microcontrollers originally.)
PCI could be hot-pluggable actually. It requires a lot of fiddly bits though. Especially, you need to setup enough address space for the cards without knowing what they are.
I have fried a PS/2 port before by doing that. Once, in the 1990s, at work. That doesn't tell you how often it happens though because I haven't hot plugged in a PS/2 device since then (and now I don't use them much anymore).
Personally, I've been lucky. Though I have seen sparks while plugging while powered. Also I've got many broken systems where at least one port (of two) was broken, but could be compensated by 'Y-wire' where you'd put mouse and keyboard via that Y-wire into the remaining port. Which later became some sort of pseudo-standard, cost cutting, because only one port is cheaper? I also seem to remember advertising on the package of better boards, which had protection against that.
FWIW, I never followed that advice for anything as far as I can remember (save for really obvious stuff like ISA or PCI cards, or in professional settings where I was following some procedure), and nothing ever blew up. Could have been lucky.
(Super-IO was very late, by the way, it was 8042s and derivative microcontrollers originally.)