The user stylesheet version is deliberately blunt, done in two rules to forcibly override whatever the author may have done, to both the normal and the interacting experience.
If you control the site’s styles, it’s better to keep specificity as low as reasonable (which I decided meant one pseudoclass—you could reduce it to zero by shifting the `:where(` to the start, but I decided not to for some reason), and only change what you need to change (which is the text-decoration-color—notably, not the entire text-decoration—on links that aren’t being interacted with.
Why? Because as well as being shorter and more conceptually elegant (and frankly, I place likely-disproportionate value on it being just one declaration, pride probably factoring in), it makes it easier to override when you want to. Suppose, for example, you want <a class="button"> to not get underlines because you’re making it look like a button (for better or for worse): you can write `.button { text-decoration: none; }`, just like you would have done previously, and it’ll work fine. Against the user stylesheet approach, even stripped of its !importants, you’d still get an underline on hover until you increased specificity (e.g. `.button.button`, and I’m assuming this is coming after the :any-link:is(…) rule, or else you’ll want `.button.button.button`) or reduced its specificity (which I grant :is → :where can do).
