• Only 0 to 256 worlds are currently supported (we're working on it)
• Iris should work just as it did in 1.16, this is simply a compatibility update to make upgrading your server a bit easier
• Removed Smoothiemaps (and all the warnings with it)
• BlockData fixes
• Iris now uses Gradle

Please read the description of the spigot page again! If you are still using 1.16 or below, use 1.3.9. Future versions of Iris will only support 1.17 and up!