The compact formatter is implemented on top of the default formatter, and it omits the failure reason YAML block. Configure the --format option to use this format:

--format RSpec::TAP::Formatters::Compact

The generated report for String spec:

TAP version 13
# test: String {
  # group: #blank? {
    not ok 1 - returns false
    ok 2 - returns true # TODO: need to implement blank? for NilClass
    1..2
    # tests: 2, failed: 1, pending: 1
  }
  # group: #present? {
    # group: when whitespaces only {
      ok 1 - returns false
      1..1
      # tests: 1, passed: 1
    }
    # group: when nil {
      not ok 1 - returns false
      1..1
      # tests: 1, failed: 1
    }
    # group: when whitespaces and other characters {
      ok 1 - returns true
      1..1
      # tests: 1, passed: 1
    }
    1..3
    # tests: 3, passed: 2, failed: 1
  }
  # group: #squish {
    ok 1 - squishes # SKIP: it is Ruby not Rails
    1..1
    # tests: 1, pending: 1
  }
  1..6
  # tests: 6, passed: 2, failed: 2, pending: 2
}
1..6
# tests: 6, passed: 2, failed: 2, pending: 2
# duration: 0.011382 seconds
# seed: 15065