"The negative voltage requirement of RS-232 is why some ATX power supplies still provide a negative voltage reference on one of the pins. The power supply manufacturers want to provide it in case the end user has a motherboard that uses on board RS-232 (some industrial motherboards still use it). usually the available current for the negative voltage reference is pretty small though (around 1 amp or less)"
I have a pile of 5 ATX power supplies of various vintages up to recent. They all had a -12 V supply. Three were 0.8 Amp. One was 0.3 Amp and one was 0.5 Amp.
Yes, there are low current requirements, so typical uart to rs232 transceiver ICs include the electronics necessary to provide the higher voltage +- supplies.
"The negative voltage requirement of RS-232 is why some ATX power supplies still provide a negative voltage reference on one of the pins. The power supply manufacturers want to provide it in case the end user has a motherboard that uses on board RS-232 (some industrial motherboards still use it). usually the available current for the negative voltage reference is pretty small though (around 1 amp or less)"