JME snapshots are now available on
https://s01.oss.sonatype.org/content/repositories/snapshots/org/jmonkeyengine
Thanks so much, @sailsman63
Edit:
For using snapshot version add
repositories {
maven {url 'https://s01.oss.sonatype.org/content/repositories/snapshots/'}
}
and use 3.7.0-SNAPSHOT
for jme version. For example:
implementation "org.jmonkeyengine:jme3-core:3.7.0-SNAPSHOT"