The case of the ever-shrinking D score. About 10-12 years ago, the Yurchenko 1.5 was among the most common vaults in international competition. Teams (at least, outside the big four) would bring athletes to the world championships and the Olympics specifically because they had a Yurchenko 1.5 to use for the team score, and Nastia Liukin won the 2008 Olympic all-around with a Yurchenko 1.5, still probably the best one every performed.
Yet now, this vault is moving toward the elite scrap pile. We do still see it of course (all-around qualification at the world championship in 2018 brought us eight gymnasts performing Y1.5s), but as difficulty expectations have progressed, the 1.5 is increasingly less competition. A major change in the pervasiveness of the 1.5 also came in 2009, when the value was dropped two tenths even though the rest of the Yurchenko layout vaults stayed the same. It stopped being obviously more valuable to do the 1.5 than the full because the full was worth only three tenths less. Now, it’s very common to see gymnasts progress straight from the full to the double and not bother with the 1.5.