I use t4-bs5-blank template on a Joomla 5 website that has a plugin that currently still uses mootools. I have found that the T4 Framework has a JavaScript file: ~/plugins/system/t4/themes/base/js/base.js which has a section of code:

// Add missing Mootools when Bootstrap is loaded
(function($)
{
    $(document).ready(function(){
        var bootstrapLoaded = (typeof $().carousel == 'function');
        var mootoolsLoaded = (typeof MooTools != 'undefined');

        if (bootstrapLoaded && mootoolsLoaded ) {
            Element.implement({
                hide: function () {
                    return this;
                },
                show: function (v) {
                    return this;
                },
                slide: function (v) {
                    return this;
                }
            });
        }
    });

This implementation of empty Element show(). hide() is replacing the implementations that come with the plugin. I have worked around this by changing the above code to:

// Add missing Mootools when Bootstrap is loaded if not already defined
(function($)
{
    $(document).ready(function(){
        var bootstrapLoaded = (typeof $().carousel == 'function');
        var mootoolsLoaded = (typeof MooTools != 'undefined');
	var mootoolsShowDefined = (typeof Element.show === "function");

        if (bootstrapLoaded && mootoolsLoaded && !mootoolsShowDefined) {
            Element.implement({
                hide: function () {
                    return this;
                },
                show: function (v) {
                    return this;
                },
                slide: function (v) {
                    return this;
                }
            });
        }
    });

Not sure if there is somewhere I can place this code in some custom file to avoid it being overwritten on the next T4-Framework update, or if something similar could be considered for the next release?