Was it common practice for pirates to replace a lost hand with a prosthetic hook? Or was this lore perpetrated by fiction and Hollywood?
It does seem like a reasonable choice, since a hook is a rather cheap and simple to make prosthesis. But is there historical evidence that shows that a hook was the go-to prosthesis for pirates? Or did their job actually require a more sophisticated prosthesis?
Apparently it's true:
From "How Stuff Works"
Most famously attributed to seafaring pirates, peglegs with wooden cores and metal hands shaped into hooks have actually been the prosthetic standard throughout much of history. While Hollywood has exaggerated their use of hooks and peglegs, pirates did sometimes rely on these types of prostheses. The required materials for these devices could be scavenged from a common pirate ship; however, a trained doctor would have been rare. Instead, the ship's cook typically performed amputation surgeries, albeit with poor success rates.
Christopher Newport is a proper historical example of a sea captain who lost an arm and replaced it with a hook. He was apparently the real life model (or one of them) for Barrie's Captain Hook. Newport was technically a privateer, not a pirate, but I think the difference is really just that a privateer was a thief who sometimes paid his taxes.
http://www.captainchristophernewport.com/
So, it is categorically not just a myth, although how widespread the practice was is hard to say. I imagine not that many pirates lost their hands, but it makes a good story.
