https://www.showcaves.com/english/index.html

https://www.hmdb.org/

https://stallman.org/

Although I see someone has put a 1.5MB image at the top, whose intrinsic size is 2000 × 2588 px, but which was downsized to 320 × 400 px. That's not prioritizing function.

https://www.compuserve.com

http://www.catcam.com (not even https)

But yes, good designs are not flashy, e.g. I love the design of Astro Starlight ( https://starlight.astro.build/), a starter kit for documentation pages.

So I also took inspiration from "simple designs" for my personal site: https://bryanhogan.com/

If by "old" you mean "minimally styled" then there are plenty of sites from that era that were really extravagantly styled, since it was a new medium that many people were exploring. There were also plenty of sites with Java or Flash that were considerably more intrusive than sites today (not to mention the period of time between when someone realized you open as many popups as you wanted and when popup-blocker plugins appeared).

Also, this is probably me getting old, but https://ucjeps.berkeley.edu/ looks quite modern to me.

https://chroniclesofgeorge.com

I eventually added proper css, bolted on https, and updated the html to something a little more modern and standards-compliant, but the site is still hand-coded, and looks pretty much the same as it has for a quarter-century.

https://www.vannattabros.com/dozer.html -- A detail page from the site, not well organized but so much great info about heavy equipment and logging.

It's fast to navigate and order parts from, works on every browser I've ever tried it in, and loads very fast because there's minimal unnecessary components to the entire site. I hope they never change it :)

https://www.unknowncheats.me/

I guess it's this one https://solar.lowtechmagazine.com/

Also like the style of Japanese websites where they seem broken/don't expand to fit available screen but cool aesthetic still

http://www.ccru.net

I doubt it's currently maintained, but these esoteric sites are fun

My last use case for it was selling a car and giving away some free stuff. Sadly, those have been replaced by fb marketplace.

I rescued the domain after it was left to expire and did my best to honour the original design from 2000.

check out their directions page: https://sokogakuen.org/info.html