There have been previous threads on the benefits of regularly changing your mumsnet username for privacy purposes because the "advanced search" feature and Google were always able to pull out all posts by username enabling information to be compiled. But now it seems the Mumsnet door has been opened to AI. If you ask ChatGPT to summarise all posts from a specific user on a specific topic, it will now do it (I think it must be recent as when I tried it several months ago it didn't work). I expect it will also help you to find cross-thread examples of posters contradicting themselves, being hypocritical, and help you find holes in their arguments. But if you're a stalker it might help you put together clues from different threads to work out where they live, which schools their children go to, etc etc.
The only protection against this is to change your username regularly. (Or possibly Mumsnet Premium provides some protection too? I haven't checked).