We have the 98 theme that is MOSTLY stable for old browsers, but reading their
info page.. firstly I find their website really confusing and lacking in almost any info explaining how the whole thing actually works, but I thINK, its doable. Though they have a status page:
https://protoweb.org/wiki/service-development-status/ it says that POST requests are not fully working, so no forum will work until they get those sorted. Also I have no idea what their "HTML" service is, verses a "proxy-pass-through", why would POST requests not work over a proxy?? Wouldn't a proxy just proxy to this server? Also if this site is up as HTTP, why would it even need a proxy?? It would just work normally
Anyway, You could make the basic forum work, I think that's doable, but everything around it would be an issue. For example the everyone site is mostly CSS that is too new for Netscape, Guilds all use new CSS, people use new CSS on the profiles etc none of that would work correctly. Anything script based would not work, so the shoutbox would not be there.
The boards and threads themselves though could function since they are totally theme-able and don't use standard layouts based on modern CSS. Anything embedded like videos would all be broken, but we have the image proxy and that could be made to work I think.
All that said... that's a lot of work

And that's work that's not going into other parts of the site and other projects, and only benefiting a few people who want to connect very old computers to the internet. I hate being too much of a pragmatist, but my thought would be, there are better things to use that effort on

If people really want a super basic read-only version of the forum that works on old computers though let me know, its not impossible to do