Even Microsoft will not support IE11 (or legacy Edge) in any way from next year, including in its own products like Teams. Other major sites like Twitter are starting to drop any support too. It won't be long until you'll struggle to use the web on IE11. Technically we have not supported it for a long time, but because we held back on modern CSS features, it still mostly worked. For 4.5, we decided now was the time to press ahead with improvements and drop support ourselves.
By dropping support, we've been able to use newer CSS features that benefit designers (and therefore site owners), simplify our theme (with more improvements to come), and we'll also be able to begin the process of modernizing our javascript.
If your community has a significant percentage of IE11 users due to your audience, you may prefer to stay on 4.4 (though that, understandably, has drawbacks too). If you're just an individual IE11 user, I would strongly suggest using a newer browser - literally any of them would be a better choice.