Apple has released macOS High Sierra 10.13.5 bringing support for Messages in iCloud.
The macOS High Sierra update improves the stability, performance, and security of your Mac, and is recommended for all users.
This update adds support for Messages in iCloud, which lets you store messages with their attachments in iCloud and free up space on your Mac. To enable Messages in iCloud, go to Preference sin MEssages, click Accounts, then select "Enable Messages in iCloud".