Komplete Kontrol S88 power supply socket size?

I agree, you should be unplugging (or turning off) the supply from the wall socket/plug end rather than the low voltage keyboard end. It's not advisable to leave PSUs powered up with no load.

"not sure where you get that info from"

How about over 40 years of electronics experience? Manufacturing, testing, repairing.

From Google:

Running a power supply with no load, also known as operating it with an open circuit, can be problematic and may not be safe or recommended for all power supplies. While some modern, reputable power supplies are designed to handle no-load operation, others may exhibit issues like overvoltage, undervoltage, or significant ripple

Don't want to stir up a massive debate over something rather trivial but seems more something google suggests based on hearsay than anything grounded in actual reason.

Now, I was not suggesting people "leave things plugged in when not in use" (tho 90% of our appliances in our homes are left connected to power when not in use…), simply suggesting that the claim it can cause damage if disconnected from the load does not sound correct to me. No one is pulling all their plug packs out the walls when they disconnect their devices (chargers for instance) and many appliances physically disconnect the power from the circuit via the power switch which is technically the same thing so logically, that makes no real sense. Sure, won't hurt to be over cautious but leaving a modern plug pack plugged in when not connected to anything is not going to cause any damage, it's not like it's building up pressure/heat or something, maybe just consume 0.2w of power for the standby circuit.

There are however cases from decades gone by which may be where you are getting the info from where iron-core transformer driven power supplies, especially ones potted in epoxy (like the old Commodore/Amstrad computer supplies) that could indeed continue to heat up even when unplugged. These were terribly cheap supplies which would run noisy and physically vibrate/hum and this would build up over time and was 100% needed to be unplugged when not in use. I have had several fail due to being left plugged in, even an old model railroad rheostat power supply actually go bang due to overheating when powering nothing, but most modern supplies are switch mode and have no such issues. In fact, there are more problems via things like overcharged/undercharged battery packs than power supplies these days.