How to output simple keys regardless of length ("128 chars hardcoded limit")

1. How to output YAML (from Ruby) with simple keys regardless of length?
2. What is the rationale for the (hard-coded) limit of 128 chars? (Include reference to standard)

require 'psych'

%w[
  https://example.com/this-url-shy-of-one-hundred-and-twenty-eight-characters-causes-libyaml-not-to-prefix-it-with-a-question-mark
  https://example.com/this-url-longer-than-one-hundred-and-twenty-eight-characters-causes-libyaml-to-prefix-it-with-a-question-mark
].to_h { [it, it.length] }.to_yaml(line_width: -1).then { puts it }

---
https://example.com/this-url-shy-of-one-hundred-and-twenty-eight-characters-causes-libyaml-not-to-prefix-it-with-a-question-mark: 128
? https://example.com/this-url-longer-than-one-hundred-and-twenty-eight-characters-causes-libyaml-to-prefix-it-with-a-question-mark
: 129

https://github.com/yaml/libyaml/blob/master/src/emitter.c#L1187

yaml_emitter_check_simple_key(yaml_emitter_t *emitter)
<snip>
    if (length > 128)
        return 0;

Start with src/emitter.c at yaml_emitter_check_simple_key(), especially the 128-character check, and reproduce the behavior using the Ruby/Psych example in the issue. Check the YAML standard reference relevant to simple keys and document the rationale and whether libyaml provides a way to emit these keys without the question-mark indicator.