Over the last short while I've seen Lightbend announce ceasing support for Play, others express that Slick is likely abandonware, and also seen my chosen Play authentication library, silhouette, get archived on github. All of this has impacted my favorite side project that I've been slowly plugging away on.

Silhouette is probably just because of one guy's schedule, but Play/Slick seems like after-effects of lightbend's layoffs many months ago, and while I don't know the cause of those it seems plausible it's because of the kind of thing you describe above.

A tiny drop, an endless collection of guest language that goes back to Beanshell, many of which now forgotten.

If people left Java for other language(s), wouldn't that leave Java with a decaying ecosystem?

Maybe worry about leveling the playing field first? There's so much money and research and development going into Java that there's no possible way for Java to end up with a "decaying ecosystem" inside the next twenty years. It's like pointing out DuckDuckGo and worrying about Google.

Most of them stayed somewhere on the jvm though. Still in the ecosystem.