Every Hampton I've been to in the last few years has similar signs, along with most Doubletrees and HGI's.

It's a nice little perk, except, of course, the hotel has no real way to monitor whether folks in those spots are actually Diamonds.

So, no, I don't think it's actually a new perk and the "reserved" status of those spots depends on a) non-Diamonds not parking there; b) there not being too many Diamonds for too few spots.