Do we know that they didn't have some backend code handing the editing?
I don't think a web where every page is globally editable by default would be a good idea, but I can't imagine at all how it would work without a backend, unless all of the changes are just local. But that seems pointless.
Being able to change stylesheets, disable or enhance various JavaScript scripts, add notes and annotations, and other things, is exactly the idea of a user agent.
The user makes a request, and then does whatever they like with the answer. Not just whatever is sensible, but whatever they want to do.
If that concept somehow became accepted again... I think the accessible web might well become a solved problem, rather than an endless slog.
You'll need to do a bit of work to make it the way it used to be. Editing any text on a page, or having your changes save persistently, needs a bit of a... Framework, to keep things together, rather than being the expected mode of interaction.
Sure, I can add a p to the tree. But if I refresh, its gone. I'll probably need plugins to keep my own stylesheets and JS changes around.
I'm aware of plugins. But these used to be builtin features. Developers needed to work with them, rather than making it harder and harder to use them to make the users life easiee.
I don't think a web where every page is globally editable by default would be a good idea, but I can't imagine at all how it would work without a backend, unless all of the changes are just local. But that seems pointless.