Since !DATA was added to the MPD spec, it turns out to be quite hard to track down. If you look at the Documentation page, it isn't mentioned, so unless you know in advance that it is in the MPD spec, it's hidden. (And since "data" is so generic, a Google search for LDraw data doesn't even include the MPD spec on the first page of results.)
I would suggest adding a parenthetical to the MPD link indicating that !DATA is included there.
I would suggest adding a parenthetical to the MPD link indicating that !DATA is included there.