It's not true. Data is only encrypted automatically on a Mac with Apple silicon or an Apple T2 Security Chip. That is to say, on your 2017 Intel MacBook Pro, FileVault will not be enabled automatically. So no need to worry about the issue of disk automatic encryption. It's up to you to enable FileVault or not.
However, FileVault is designed to increase the security of your data by stopping unauthorized decrypting or accessing without entering your login password. It is suggested to enable FileVault to protect your data. For more details, please refer to What Is FileVault And How to Encrypt Disk with It.
Before updating your macOS from Mojova to Monterey, you need to do some necessary preparations, so as to download the macOS Monterey installer successfully.
- Make sure your Mac has enough disk space.
- Check if the network connection is secure and stable.
- Keep your Mac charged during the updating process.
Furthermore, always back up your Mac beforehand to avoid possible data loss. You can use the macOS built-in tool, Time Machine, to back up critical files even the whole startup disk. Or if you prefer a faster and easier backup experience, iBoysoft DiskGeeker would be a good fit.