WebStorm 2018.X uses Java 8 by default, and 2019.2,x uses Java 11.
In regards of our tools there are no changes that leverage features coming from WebStorm, so if you want to use the latest features that WS offers but you need SDF you could downgrade the JDK version. If on the other hand, you use WebStorm just for NetSuite customization and you don´t use/need the latest enhancements from the IntelliJ tool, then you could stay in 2018 (or even in 2019.1 without making downgrades).
In the upgrade in which we are working, we are considering the WebStorm enhancements to be compatible with our plugin (is not only the JDK version).