If somebody is still interested in 2023.
This code works with JOSM Scripting Plugin 0.2.7 using the latest API V3.
It is a rough replacement of the JOSM tool “split way” (keyboard shortcut “P”)
Code is not fully tested, use with care.

import josm from 'josm';
import { DataSet, DataSetUtil } from 'josm/ds'
import * as util from 'josm/util';
import * as command from 'josm/command'
import * as console from 'josm/scriptingconsole'

const layer = josm.layers.activeLayer;
var ds = layer.data;
var dsutil = new DataSetUtil(ds);
const sel = ds.selection;

const selection = dsutil.query("selected");
const nodes = dsutil.query("type:node selected");
const ways = dsutil.query("type:way selected");
const myWay = ways[0];

const SWC = Java.type('org.openstreetmap.josm.command.SplitWayCommand');
const chunks = SWC.buildSplitChunks(myWay, nodes);
josm.console.println("chunks: " + chunks.length);

var ArrayList = Java.type('java.util.ArrayList');
var list = new ArrayList();
list.add(selection);

var result = SWC.splitWay(myWay, chunks, list , SWC.Strategy.keepLongestChunk());

const UndoRedoHandler = Java.type('org.openstreetmap.josm.data.UndoRedoHandler')
const split = UndoRedoHandler.getInstance().add(result);
josm.console.println("Split done.");