Just a quick post since I speak with many network engineers and I notice some confusion about this topic.

Are all 4 pairs of an Cat5e cable used?

The answer is.. it depends.

For gigabit speed 1000T all four pairs are used.

If 100TX speed is enough we can of course split the cable and double the connections without pulling more cables:

In my specific case I’ve used a single cable drop in the garage to connect a humidity/temperature to a 1wire bus sensor while allowing a future connection of an Ethernet device.

HTH