This is the key date to the $2.5 Indian series and is one of the more famous key dates of the 20th century. Decades ago (1950s and 1960s and before) everyone viewed coins based on mintages. The 1911-D has an original mintage of only 55,680 coins. The next lowest mintage is the 1914 with 240,000 minted and average mintages for $2.5 Indians is in the 400,000 to 500,000 range. So the low mintage of the 1911-D certainly was easy to notice. At the time, the 1911-D was way more expensive in all grades than any other $2.5 Indian.
Nowadays, we realize original mintage is not the only factor in rarity and we focus more on survival estimates. And we have better tools in population reports and expert survival estimates, which are now very comprehensive on PCGS CoinFacts. For the 1911-D $2.5, its status as a rarity has held up. It is still the rarest $2.5 Indian and it is still way more expensive than the other dates in the series in circulated grades. It is also the most expensive $2.5 Indian in mint state grades. There are a handle of Gems known, including several superb MS66s.
Note that there is one variety of 1911-D $2.5 that has a very weak (barely visible to say the least) mint mark. Though rarer than "strong mint mark" variety, it sells for much less. Seems that when someone buys a 1911-D $2.5 they actually want to see the D!