Following through on a New Year's resolution to re-read 'The Go-Between' after 26 years (thoroughly enjoyed it, btw), I came across the curious word "ha-ha". I don't have the sentence verbatim, but the context was that there was a ha-ha separating the garden from the forest.

M-W tells me that a ha-ha (from the French haha) is a "sunk fence" (ugh! not sunken fence?), which I gather is a fence in a ditch.

My questions are:
1. Is this word still in use?
2. Is it a regional (UK?) thing?
3. Do sunk fences still exist? I can't recall having seen anything, anywhere, that would qualify for the description.