That's what https://yaml-multiline.info is for! I know the author's on HN, I hope they chime in here :)

Amusingly, the Stack Overflow answer you linked in your contribution is the second result in a Google search for "YAML multiline string" or the like, after yaml-multiline.info; the two combined appear to the canonical resource on the web.

Heh, not sure if you realise, but that website was inspired by my SO answer. (I'm credited at the bottom).