Some things going on here:

If I have a gemspec saying target_ruby_version ">= 2.6", what RuboCop will actually use is 2.7 because it is the default version. All versions prior are simply discarded, even if rubocop is able to provide accurate results for these older versions.

There were tests for this but I believe that they lost what they actually were testing against during some refactor, somewhere here: #9515, #10629, and the PRs removing and reading analysis for older rubies.

This test for example used to check that the returned ruby version is 2.4 and not 2.3 which it didn't know how to analyse at one point (i.e. don't select versions below what we are currently supporting):

          it 'sets target_ruby from required_ruby_version from inclusive requirement range' do
            content = <<~HEREDOC
              Gem::Specification.new do |s|
                s.name = 'test'
                s.required_ruby_version = Gem::Requirement.new('>= 2.3.1')
                s.licenses = ['MIT']
              end
            HEREDOC

            create_file(gemspec_file_path, content)
            expect(target_ruby.version).to eq default_version
          end

Now it checks that 2.7 is the returned version, which doesn't make much sense since older versions are able to be analysed.

For the < 3.0.0 case, I think its ok to select the earliest version supported (currently 2.0). Selecting 2.7 (or whatever the default may be in the future) surely isn't the intention.

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.