Saves are never deliberately deleted, and are always forward compatible with new versions.
So if your save is getting deleted, it's because your browsers data is getting cleared. You can either download the desktop version, which stores saves in files in your user data, or use the Import/Export menu to export your save and store it anywhere you like.
Also, if you ask on the Discord, someone is usually happy to send a save file to help people get back to where they were. If you'd still rather wait, that's fine. I just wanted to explain what happened to your saves and give you some options.