SirPrimalform: The internet is a constantly changing thing. Why you would expect an old browser to keep working is beyond me.
Cavalary: Because it's not somebody's personal site to work according to how that person wants and what they cand manage to do, or one maintained by a skeleton crew of volunteers that's limited to what the volunteers may offer in terms of coding and maintenance, or even a fully free service where you may say that such conditions are the price of admission, but a commercial site, a store, which should aim to maximise ease of access and appeal to any and all potential customers. Plus, they did start out by catering to those interested in old games and who in general take issue with current standards and trends, and some of those people are still around (despite GOG alienating them with just about every move they made for the past 5 years...), and they definitely still have old games, so some may even have old computers with old OSs to play those old games, so it'd make sense to also visit the site and make any such purchases and download the installers on those computers they intend to play the games on too.
We're talking about the internet here, not whether GOG sells old games or not. GOG's target demographic is irrelevant, outdated browsers are outdated browsers.
When I first started using the internet in the late 90s, I was using AOL's dreadful browser. It worked back then (just about), but I wouldn't expect it to work now. Internet Explorer is a dead browser and Palemoon is wilfully non-compliant.