Hello @Nakashin
Thanks for setting up the request. That saves a lot of back and forth.
The macOS 12 requirement is new. Roon Bridge moved to .NET 10 on all platforms with 2.71 build 1683, and that release brought a new minimum of macOS 12 for Mac. It was announced on August 31:
So there is no Catalina-specific installer as such. Roon Bridge ships as one macOS installer, and this release raised the floor beneath it. Installing an older version will not solve the problem because the bridge will not connect to the Roon server now or later. Our guidance in that announcement is to check whether Apple offers a system update for your Mac, and if it cannot be updated, to move Roon Bridge to a supported machine.
In your case the first of those applies. Your MacBook Pro 13-inch Early 2015 is on Apple’s supported list for Monterey, so the machine itself is not what is holding you back.
On the third-party download sites, your instinct is right. Please do not install a Roon binary from one.