most websites just check the browser useragent, and if you spoof the useragent, it works. most websites are blocking it artifically even if the website works fine with your browser. so i think it's worth a shot if there are chrome plugins who can spoof the browser useragent.
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".
yupp, and i hate that. i use a firefox version that don't supports private fields, and because a common js lib uses them a lot of websites suddenly stopped working for me just because of this bs. instead of just using a normal variable they use private fields and kill a ton of older browsers by doing so. and most website owners don't care so asking them just leads to them saying "just upgrade bro".