I am using Horizontal Timeline JS https://cdn.jsdelivr.net/gh/ycodetech/horizontal-timeline-2.0@2/css/horizontal_timeline.2.0.min.css and Bootstrap Version 5. Used https://polyfill.io/v3/polyfill.min.js still not working on Internet Explorer.
You did not inform us what is exactly not working or whether you are getting any error while doing a specific thing.
I refer to the Horizontal Timeline documentation to check its compatibility with browsers.
If we refer to the second point, we can notice that IE 11 has issues with the Timeline and it will not going to fix.
So this could be the reason that why your Timeline code is not working with the IE browser.