Looking back at all prior HN stories with "hacktoberfest" in the title or with "hacktoberfest" and "spam" in the comments, I can't find any that talk about it being a big problem before this year.

I found one maintainer who said they got a non-negligible number of garbage pull requests for it, but they still said Hacktoberfest was cool. There was also one post about a filter one project adopted a year ago that used heuristics to identify and automatically reject Hacktoberfest pull requests from new submitters that were too trivial.

I'd have expected more if it was a problem to the extent that it seems to be this year.

Malicious bots would be 2nd.

I've never learned why DO is a malicious traffic leader. Maybe they're competing with OVH to be the next MCColo.