From https://github.com/checkstyle/checkstyle/wiki/Checkstyle-GSoC-2021-Project-Ideas#project-name-upgrade-java-grammar-from-antlr2-to-antlr4:

Checkstyle needs to have new Java grammar that is based on ANTLR4 version. This task is very difficult but it is critical for Checkstyle as ANTLR2 library is not supported (from 2006) and is far less efficient. Old version has a bunch of syntax analysis limitations that have already been resolved in ANTLR4. Our team is already experiencing difficulties with support of current grammar as it is too complicated due to limited parsing abilities of ANTLR2.

In this issue, we can discuss finer points of this upgrade and remaining issues from https://github.com/checkstyle-antlr4/checkstyle-antlr4:

Grammar should use all caps in naming of literals

Criteria for main repo merge

Add EOF token to AST