Yes. They will replace the Rules DSL as the default Rules Engine. The plan is to write a module that will allow Rules DSL Rules to execute on the NGRE.
No. As of right now they are the future.
It is important to make a distinction. Rules DSL, as it exists right now is completely separate.
Then there is JSR223 and the NGRE. Both use the same execution engine. But when referring to JSR223 we are usually referring to Rules written in Jython, Javascript, or Groovy in text files. When referring to NGRE often we are referring to PaperUI created Rules which get stored as JSON in the JSONDB (right next to Things and Items). These Rules support a significantly different structure and approach to Rules writing (see Experimental Next-Gen Rules Engine Documentation 1 of : Introduction).
They are Experimental because the NGRE are not well documented yet nor are they complete (missing some important Rule triggers). JSR223, which run on the same execution engine, are more mature though the documentation is hit and miss depending on which language you choose. But since they use the same execution engine to execute, I see no reason why JSR223 won’t be supported for the foreseeable future.
That being said, anything can change at any point. New technologies are invented all the time, major problems encountered that are insurmountable, technologies are dropped, etc. For example, if Java 15 (or some version) decides to drop support for JSR223, we will probably have to find an alternative.