Am I correct in understanding that if I want to use Parinfer in Spacemacs, I need to checkout the pull request https://github.com/syl20bnr/spacemacs/pull/7339 ?

I.e, Parinfer hasn't made it into "official" Spacemacs, and there aren't plans to do so?

It is in the devolop branch…

I can only recommend to switch to develop.