custom-element-15.js

...
this._disconnectedObserver = new MutationObserver(mutations => {
    mutations.forEach(mutation => {
        if (mutation.type === 'childList' &&
            Array.prototype.slice.call(mutation.removedNodes).indexOf(this._el) >= 0) {
            ...
        }
    });
});
this._disconnectedObserver.observe(this._el.parentNode, {
    childList: true
});
...