Hi Foo,
Since Play is an open source project, there's no such thing as end of life - as a last resort, you can always fork it, backport a fix to any version, and cut a new build for yourself, so in that sense, every version of Play can always be alive. Many people do this, it's one of the greatest advantages of using open source software.
There likely will be a Play 2.3.10, but 2.3.x releases won't happen as frequently now that 2.4.x is out. As a general rule we don't backport fixes to older releases, unless it's a critical issue or security issue for which there is no reasonable work around. If you need long term support for a release, then you should consider purchasing a Typesafe subscription - using Play under Typesafe's commercial terms gives you a minimum of 2 years from the release date of each major release of Play before end of life.
Regards,
James