See the release notes, which provide a link to download the previous version:
Going forward, a free option is to update your Mac with the OCLP app. This works with nearly all Intel Macs at least to update to macOS 12, which is all you need.
We’re sorry to hear you’re temporarily without Roon. As others have mentioned, this is a result of the updated minimum requirements for running Roon.
You have a few options to get things running in the meantime:
To run the server on an older Intel Mac, you’ll need to use a third-party app like OCLP to update to MacOS 12.0.
If you’re only running Roon Remote on this Mac, you can use a temporary download link to keep the older, compatible build. You can find the installer here.