I don’t know what the cutoff is on that. Here’s what @geek actually told me:
Most of the time people either need to check their spam folders or they got automatically demoted (if you don’t open the email for a few weeks we stop sending to you).
So you’re not going to stop getting newsletters if you skip it for 3 days. If you skip it for a month, you may end up not receiving it any more.
And there’s always the possibility that the analytics are flawed. I don’t know what the specific item is that triggers this check; it’s possible that certain privacy-enhancing browser addons could be interfering with the analytics.
Did you try to re-subscribe?