No, Java 8 doesn't stop getting support until December 0f 2020, and even that has an asterisk by it, as it could get extended further.
As for when, as soon as possible. I don't have a roadmap for you although it probably won't be until later in 2019.
Whan will sqldeveloper be usable with java 11 (10 is already end of life)?
Oracle does NOT pre-announce, confirm or promise either release dates or functionality.
Just check the download page for the sql dev version available and the JDKs supported.