> Is java able to stackify most new Object calls and elide default initialization of object members these days?

Escape analysis in OpenJDK will stack allocate values where it can show it is safe to do so. Project Valhalla is also reducing the memory footprint of objects.

As for thread safety, that is more of a language concern than a runtime one. Amongst JVM languages Scala is leading here AFAIK. Its "capture checking"[1] provides thread safety (e.g. [2]) and actually covers escape analysis as well. On Scala Native (the native code backend for Scala) capture checking can be used for safe stack allocation and safe arena allocation.

[1]: https://docs.scala-lang.org/scala3/reference/experimental/cc... [2]: https://softwaremill.com/understanding-capture-checking-in-s...