Is it my imagination, or did the Explore criteria go up for DYS?

No, you are not imagining - definitely went up recently. I have not taken the time to do a side by side comparison, but in some areas - it may have gone up rather significantly. It would appear that a minimum composite number, however, is no longer necessary.

I am wondering if the EXPLORE test got easier over time, or if there is another reason for this change. Especially interesting to me since I thought the test was being phased out.

Just wonder if we should be reinterpreting scores from recent years?

Well, from what I understand, the test itself will no longer be given after this year, since it will be replaced with the Aspire.

I have wondered, though, if factors such as "self-selection" (or "parent selection"), increased test preparation, and some students' exposure to above-grade level material have raised scores over time.

Good point. I did not realize until recently that a parent could just sign their child up. In our case, the AT teacher sent the invitation out only to the kids already identified as gifted.

Previous threads include What can we use now that EXPLORE is gone?

It's often helpful to see the source of information, in this case the test material companies offer links such as: http://www.act.org/products/k-12-act-explore/

The cut-offs for Explore did seem disproportionately low to me vis-a-vis other accepted tests when I looked at the numbers a couple of years back. Davidson must have access to a ton of cross-data from both DYS and applicants. Maybe they finally noticed that the old cut-offs did not correspond to the 99.9 in the other tests.