Edit: found it, there's mention of it in the Participating subpage, in a couple of subsections (including "SPAM"). However, it still doesn't look too promising, since it seems to put the burden of flagging it as SPAM on the maintainers:
> "PR/MRs that are labeled with a label containing the word “spam” by maintainers will not be counted. [...] PR/MRs that our system detects as spammy will also not be counted."