You know, if I've noticed anything in the past couple years, it's that even if you self-host your own site, it's still going to get hoovered up and used/exploited by things like AI training bots. I think between everyone's code getting trained on, even if it's AGPLv3 or something similarly restrictive, and generally everything public on the internet getting "trained" and "transformed" to basically launder it via "AI", I can absolutely see why someone rational would want to share a whole lot less, anywhere, in an open fashion, regardless of where it's hosted.
I'd honestly rather see and think more about how to segment communities locally, and go back to the "fragmented" way things once were. It's easier to want to share with other real people than inadvertently working for free to enrich companies.
I don't know if there's something similar for text. You could try writing nonsense with a color that doesn't contrast with the background.
The evidence Nightshade works is that AI companies want to make it illegal.
Like most adversarial attacks, they get more perceptible as they try to be robust to more transformations of the data (both in practice, i.e. applied to a level that's non-trivially removable, tend to make images look like slightly janky AI, ironically), and they are specific to the net(s) they are targeting, so it's more of a temporary defense against the current generation then a long-term protection.