antlr-tool-version is a variable defined in `antlr-mode.el'.
Its value is

The version number of the Antlr tool.
The value is an integer of the form XYYZZ which stands for vX.YY.ZZ.
This variable is used to warn about non-supported options and to supply
version correct option values when using M-x antlr-insert-option.

Don't use a number smaller than 20600 since the stored history of
Antlr's options starts with v2.06.00, see `antlr-options-alists'. You
can make this variable buffer-local.

You can customize this variable.