inh
Re:Google’s robots.txt parser is now open source
on Tuesday July 02, 2019 @06:14AM (#20333705)
Except... they were correct.
Google has now clarified that they're removing the code behind the undocumented items, with noindex called out explicitly.
https://webmasters.googleblog.com/2019/07/a-note-on-unsuppor...
It wasn't officially supported / the recommended way - but it worked (in many cases.)