As far as I recall, the vast majority of kokonotsuba's JS never worked in any version of IE to begin with - even pre-overhaul. That includes the ability to expand images when clicking thumbnails, which cannot reasonably be done with HTML/CSS alone (not to mention that the settings window to enable/disable the feature never worked in IE either...)

A large part of teh HTML overhaul was making koko HTML5-compliant rather than the mixture of various older standards and totally invalid crap it previously was - and without relying on extremely new features or breaking compatibility just for the sake of it. Styling done with HTML tags/deprecated attributes was largely replaced with IDs/classes + CSS, so if ur browser happens to not support certain CSS features, things will look less "styled" than they used to. The essential functionality for browsing and posting should still totally work however (or at least as well as it previously did)

Another aspect of teh overhaul was utilizing CSS variables, which we decided were worth adopting despite being a "newer" feature (and by that I mean it became widely available 10-ish years ago). For compatibility with older browsers (IE9+, Firefox 6+), a script called css-vars-ponyfill was added which converts CSS variables to "regular" CSS - sadly it does not work with alternative styles as far as I could tell

The oldest browser I routinely tested teh overhaul with was SeaMonkey 2.49.5 for Windows XP, released in 2019 - I believe it's roughly equivalent to Firefox 52.9.0 ESR from 2018. 99.9% of features and styling works as expected on it except for a couple of extremely minor things (from what I remember: the SVG window close/minimize button icons are a little visually b0rked but still functional, and the "gap" property for flexboxes is not supported so a couple of things appear to be unspaced). If you run Supermium on XP instead, it's flawless