Looking at many of these now they definitely appear dated. At the time, of course, these would have been “cutting edge”
I think one clear thing we can see is a trend toward more homogenized UI on web in the last 20years.
I worked as a web dev in ad agencies in the early 2000s and built a lot of Flash sites, banners ads, and games that - like a lot of the sites showcased here - were quite unique in their design and aesthetic.
Slowly over time these started to disappear as people embraced web design trends and techniques that meant everything started to look the same.
I think a large part of this at the time was due to Flash being killed off, trends like “flat design”, frameworks, jQuery, and Wordpress becoming popular.
Marketers and designers became more savvy to what “works” online and everyone copied each other in a race for attention.
I think at least we have gradients and less muted colors coming back.
We also have way better typography than 20 years ago, and I think that's what truly makes older designs "look old". They were restricted to web-safe fonts and had to put stylized text and wordmarks into low resolution images. We have better browser support for SVGs too.