It's been changed recently (see history & killed files) and I don't want to keep documenting it: users shouldn't mess with their history file that easily (this is not an excuse for sloppy parsing, which isn't the case.) Furthermore, the format is easy, I think intuitive, and the code easily found in fs.c.

"set set-title = 1" is quite a mouthful. The old name will still be supported for a while to ease backwards compatibility.