Talking about Arduino UNO, some UNO has built-in LED (L LED permanently connected to 13th pin) in the board and dafault 'blink' program use it to blink the LED. But during other projects, many programs uses same 13th pin for other type of signal out. In this time, the signal has a 1k and a LED always in parallel. Won't the parallel LED affact different types of signal comming out from the 13th pin?
(Additional: Some Chinese clone (CH34) UNOs has no such L LED, just has empty pads on boards.)
Pin 13 is hardware SPI clock. I haven't had any trouble with SPI. But there can be trouble, like trying to declare pin 13 as led as well as SPI clock at the same time. https://forum.arduino.cc/index.php?topic=71618.0
So if you need to use pin 13 and SPI, better to disable SPI during use, just to be safe. https://playground.arduino.cc/Code/Spi
The driver inside the processor chip can source about 25mA, and sink about 40mA. The LED current limiting resistor is set up to drive the LED with about 5mA, so it slightly decreases the ability of the Arduino's pin 13 to source current while slightly increasing its ability to sink current.
Bottom line, you can treat it like any other pin...with or without the LED.