If you want to, you can set Mastodon to automatically delete your posts after a certain time period.

1. Log in through your server's website
2. Go to ⚙️ Preferences > Automated Post Deletion
3. Tick box marked "automatically delete old posts" and choose time period after which posts will be deleted
4. Choose exceptions (which posts to keep even if they are older than the time period)
5. If you are sure, click "Save changes"

If you change your mind, untick the box and click "Save Changes".