OP here: I had the same thought, but noticed a very similar trend in both [0]; I think this graph is more interesting because you'd expect the number of new users to be growing [1], but this seems to have very little effect on deleted questions or even answers

[0]: https://data.stackexchange.com/stackoverflow/query/1927371#g...

[1]: https://data.stackexchange.com/stackoverflow/query/1927375#g...

The second graph here ([1]) is especially interesting because the total montly number of new users seems completely unrelated to number of posts, until you filter for a rep > 1 which has a close to identical trend