What polyfuses protect USB ports in a typical modern PC?
Sorry, but all answers are incorrect. Vast majority of PC mainboards do use polyfuses (aka resettable fuses) to protect USB ports from overcurrent and prevent damage from melting wires and connector's pins. Below are two examples, for ASUS Z170-A mainboard

And for ASUS P9X79LE

USB specifications define that &quot;high-powered hosts&quot; must supply AT LEAST 500 mA for USB2, and 900 mA for USB3 ports (and more for Type-C). At least. General rule of design is consumer safety and to reduce OEM liability for setting fires, so the designers tend to limit the available current. High-side switches are expensive, and require even more expensive wire and extra pin on CPU or EC (or whatever). So simple polyfuses do their job quite economically.
Also, 95% (or (99?) of all usb hubs do use polyfuses as well.
As you rightfully found, maindoard use polyfuses at about double the minimum, some can supply up to the level of entire +5VSTBy limit (2.5 A and above).
If someone still has a concern about newer mainboards, here is a snippet of Gigabyte X570 AORUS MASTER (released sometime in 2019), with polyfuses circled: