Older versions of Xcode Command Line Tools on macOS Catalina
The command-line tool installer installs lots of stuff (libraries, frameworks, etc.) in multiple OS system directories.  So you probably need to find or create a volume (external drive or partition) with OS X High Sierra on it for the install to complete.

I usually partition my SSDs so that I can keep multiple bootable macOS versions on it for running such older tooling and apps.