Are there any over-current protection measures on RPis? Either on GPIO or power supply pins.
Are there differences between versions?
Unfortunately two of my 3b+ boards are dead now because of brief periods of shorting 3.3V supply to ground.
The only over current protection is on models with a polyfuse. I think all but the Pi Zeros and Pi Zero Wireless have polyfuses.
The polyfuse will limit overall current through the micro USB socket to an amp or so. There is no protection if you power through the expansion header.
There is no over current protection on the GPIO. However, in my limited experience, shorting a high GPIO to ground or a low GPIO to 3V3 for short periods (seconds) is unlikely to cause any damage. I measured of the order of 50 milliamps through a GPIO in such circumstances in my tests.
The 16 milliamp figure you will often see quoted for a GPIO is the current which can be drawn while guaranteeing the GPIO voltage will still be high enough to be seen as a logic high.
GPIO pin have adjustable current output up to approximately 16mA in 2 mA steps starting at 2mA.
However there are NO official data published covering this by Broadcom.
Here is one reputable source