I happen to have an older (2010) Mac I use as an iTunes streaming device that runs macOS 10.13, or High Sierra as it's called by Apple. A few months ago I noticed the update page found on the App Store was, well… Screwed up to say the least. Based on one of many lengthy posts on Apple's own cancerous discussion forums filled with people pissing and moaning about it, the general consensus is some idiot in charge of this stuff at Apple has broken a cascading style sheet or outright deleted it and now the 'App Store app' on older versions of macOS can't display the page properly, which has effectively broken it.
I should note at this point I've actually encountered this issue before, as far back as late last year I remember it happening to me, but quitting and relaunching the App Store seemed to correct the error. It didn't become a consistent problem until a few months ago though and now it will render like this all of the time. I've attempted deleting preferences and other files pertaining to the App Store with no success, while some people have actually resorted to completely reinstalling macOS, also with no success. It's not just High Sierra that's suffering from the problem either; macOS 10.12 and 10.11 and possibly older versions suffer from it too, though I can't confirm that.
You can still update your Mac by using the terminal though, so this isn't exactly the end of the world… There's a lot of resources on the net that tell you how to do it, but if you can stomach Reddit then this post explains it well enough. Alternatively, you can install the updates manually yourself by downloading them from Apple, but take heed that although the security updates are easily accessible from there, as is iTunes 12.8.3, some other software downloads like Safari 13.1.2 and the iTunes Device Support Update aren't available for whatever reason (though as you can see I've linked directly to the downloads for you, courtesy of this nice person's GitHub page).
Anyway, place your bets! Will Apple fix it, yes or no? Considering High Sierra is now unsupported as of Big Sur's release last year and won't get any new security updates, nor will it see any further iTunes or Safari releases, my money is on no.
Apparently I was wrong. I don't know when they did it, but after months of this problem existing, Apple have finally pulled their thumb out their arse and fixed it.