We are using ecj from org.eclipse.jdt.core.compiler in our project, however it looks like the package is not updated since 07-Oct-2016. Is it no longer maintained?
Or should I switch to the ecj from org.eclipse.jdt? Looks like the version number (3.20.0) is older than the former: 4.6.1?
The official coordinates of ecj in Maven are indeed: org.eclipse.jdt:ecj.
As you can see, this artifact exists in all release versions since 3.12.3 (corresponding to Neon.3).
Artifacts in other groups may have mistakenly used the version number of the Eclipse release, not that of ecj itself, thus appearing to be newer even if they are (a lot) older.
(When Eclipse SDK was bumped up from 3.x to 4.x this was due to breaking changes in the UI part of the code. ecj, however, remained compatible and thus stayed at 3.x)
In case of doubt, run the following to see the real version of ecj:
$ java -jar ecj.jar -version
The latest release version will answer
Eclipse Compiler for Java(TM) v20191203-2131, 3.20.0, Copyright IBM Corp 2000, 2015. All rights reserved.