Usually they’re building the website with browserlist and polyfills, and they specify how old a browser they wish to support, usually by analysing percentages of public usage, or they allow types only supported in newer browsers. Meaning if they use a feature only available in newer browsers, then it won’t be automatically backported to support older browsers.
But that’s only if they actually use those features, they’re just available to them. And it’ll only break in those places they do use them, which could be quite little of the site.
So often it’s just “we can’t guarantee it’ll work in your old browser and enough of our users use newer browsers that we’ll block you and not care”.