Had a little trouble getting ruby 2.6.6 (actually openssl 1.1.1) to install on the latest and greatest version of OSX, found a simple work around:

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optflags=-Wno-error=implicit-function-declaration asdf install ruby 2.6.6
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  WARNING: ruby-2.6.6 is past its end of life and is now unsupported.
  It no longer receives bug fixes or critical security updates.