Unless you have the control on the webpage hosting a content, it’s pretty hard to remove something out of the internet. Even if you do, people may-have back-up or screenshots, then there is place like internet archives where you’d find tons of old websites.

That said, tons of stuff end-up disappearing over-time. Look how many forum have closed down, is myspace still alive ? Recently, the French "Skyrock blog"platform closed down, but all of it’s archive have been given to the French national library so in a few decades, sociologist will have tons of data about the youth of the millenials

Some good practice of digital hygiene include :

• Do not use your real name, and change pseudonyms

• Beware of photos, at a point mass face recognition will be the norm, so assume you’re not anonymous, Posting a photo after proudly finishing a Marathon or earning a Karate black belt is fine. But if a photo isn’t fine out of context, don’t publish it.