They disappear, eventually. Long answer, TripAdvisor removes your reviews after a hotel/attraction/restaurant has been closed for a couple of years. This is concerning if you do not back-up your reviews.
TripAdvisor does not allow you to download or archive your own reviews. Taking your data with you should be a right. There are bootleg tools but they do not work well, if at all. If you do not manually back up your reviews, you are stuck.
I am lucky. I built a system to back-up my reviews. It is robust, and goes far beyond the features of TripAdvisor. What’s the best steakhouse I’ve visited? What’s the worse hotel experience I’ve ever had? What was the name of that cocktail lounge in Calgary? I know.
I’m breaking the rules and I can’t help you do the same. TripAdvisor needs to understand that online reviews are memories. Reviews are often used as a replacement for the notebooks of the past. There is value in these reviews even if they only display to the original author. It’s a shame.