Not OP, probably different numbers, but Snyk has recently published a JVM ecosystem report containing a similar question: https://snyk.io/jvm-ecosystem-report-2021/ (60% of people use JVM 8 in production, but 40% use multiple versions, so e.g. 62% use JVM 11)
It's just sort of in the air. There are several organizations that conduct and publish annual surveys asking Java people what version they're on. Jetbrains and Perforce do, and I think IBM has one, too. The numbers get tossed around in Java tech talks so often that I've long since stopped keeping track of which specific source everyone's quoting, but the figures generally fall on that range.
They may not be relevant based on how large the Java community is, but I think part of it is that the Java-targeting languages like Kotlin, Clojure, and Scala are still targeting Java 8.
Though, I would argue that many Kotlin and Scala users have a stronger incentive to migrate to Java 11 or later than Java users do. Java 8's JIT compiler and default garbage collector tend to punish you (performance-wise) for using Kotlin or Scala; Java 11 and later have made great strides to improve that.
I don't know about Clojure, for example, but Kotlin targeted Java 7 for a long time. By which I mean that the standard library didn't use anything from Java 8 or later, and the compiler would emit Java 7 compatible code by default. You can still compile it to target later versions, though.
It's only fairly recently bumped up to Java 8 being its minimum (and default) Java version.
from a general survey point of view, there are confounding aspects to a single number like that. Since Java is an enterprise language by design, the difference between 'moving versions due to management declaration' and 'our major ecosystem component supports THIS' also factor in to the overall picture.
As an outsider who does follow this for editorial reasons from time to time, most recent info (last year) was yes, projects surveyed are staying on (open) version 8.
thanks,