I can only really speak to reddit, but I think this applies to all of the user generated content websites. The original premise, that everyone agreed to, was the site provides a space and some tools and users provide content to fill it. As information gets added, it becomes a valuable resource for everyone. Ads and other revenue streams become a necessary evil in all this, but overall directly support the core use case.
Now that content is being packaged into large language models to be either put behind a paywall or packed into other non-freely available services. Since they no longer seem interested in supporting the model we all agreed on, I see no reason to continue adding value and since they provided tools to remove content I may as well use them.
I believe the effect is entirely topical, it just doesn't hurt to ingest a little too. The fluoride ions trigger re-hardening of the tooth enamel and can take the place of missing calcium in the outer enamel structure, but those only happen when they hit the outer enamel in your mouth, you don't regrow enamel on fully formed teeth