Since which version of InDesign the TextFrame duplicate method copies overset text?

That might be difficult to find out, and likely impossible from just reading release notes because tiny scripting API changes are *very* unlikely to have been included, especially back then. You would have to check each version that you could get your hands on (and get running succesfully!).

Hypothetically, if we decided that checking each version was not worth the effort, is there something we can do to help? Do you need the split story script to remove overset text? That is easy to do. I have written my own Split Story 2 script which has some advantages (mainly that you can undo it in one go!)

- Mark

/**
 * @File SplitStory2.js
 *
 * Splits the selected story into separate (i.e., unthreaded) text frames.
 * To use this script, select a text frame or some text, then run the script.
 * @7111211 m1b
 * @version 2025-07-16
 * @discussion https://community.adobe.com/t5/indesign-discussions/since-which-version-of-indesign-the-textframe-duplicate-method-copies-overset-text/m-p/15416461
 */
function main() {

    var settings = {
        // whether to remove overset text from the split frames
        removeOversetText: true,
        showResult: true,
    };

    if (
        0 === app.documents.length
        || 0 === app.activeDocument.selection.length
        || !app.activeDocument.selection[0].hasOwnProperty('texts')
    )
        return alert('Please select some text and try again.');

    var doc = app.activeDocument;
    var story = doc.selection[0].texts[0].parentStory;
    var frames = story.textContainers;

    for (var i = frames.length - 1, newFrame; i >= 0; i--) {

        newFrame = frames[i].duplicate();

        if (
            settings.removeOversetText
            && newFrame.texts[0].parentStory.length > newFrame.characters.length
        )
            // remove overset text
            newFrame.characters
                .itemByRange(newFrame.characters.lastItem(), newFrame.texts[0].parentStory.characters.lastItem())
                .remove();

        // remove the original threaded frame
        frames[i].remove();

    }

    if (settings.showResult)
        alert('Split story into ' + frames.length + ' frames.\nBe aware that this may interfere with paragraph numbering.');

};
app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Split Story');