People have suggested glitch and neocities, which are probably the closest to what you’re describing, but I’d like to add a few more that are close to but not quite what you’re asking about:

Carrd is ubiquitous among a certain kind of zoomer, and while it’s more of a no code tool, I think it’s likely to be a gateway into web design for many, and probably the closest analogy to Tumblr customization Gen Z has.

And this is a bit farther afield but if you look at the popular stuff on Scratch you get the sense that a lot of young children are using it creatively, and that it will be a huge gateway into the industry in the future.