I am still confused for days whether this is a real news or a hoax. Only a substack user saying they received this email. I did not. And there is no official statement by Substack. What is really going on here?
I've seen the leaked data posted on forums. I'm assuming they're trying to minimize the bad PR from this incident by only doing what's legally required, which is to notify affected users. They're likely not obligated to notify the broader public. Whether they should be obligated to do so is another discussion entirely.
I'm fairly sure even mentioning the name of the forum isn't allowed on HN. It should be trivial to find it yourself, though. I also replied to someone else with the CSV headers if you're only trying to find out what exactly was included in the leak: https://news.ycombinator.com/item?id=46932380
Also, keep in mind that this is a partial leak. The data was scraped from some leaky endpoint which was patched out before every user could be scraped. Only users who were in the partial leak received emails (I have two accounts, only one received an email). If you're a Substack user but didn't receive an email, I'd assume you're not in the leak. Troy Hunt should load it into HIBP eventually, and those concerned can check there if they don't want to seek the leak out on their own.
This is actually a great analogy for why companies should take small data leaks seriously. A leak is a leak.
Also, to clarify, I don't mean to appear as though I'm discrediting this leak or downplaying its severity. I only mentioned that it was a partial leak to offer an explanation as to why some users received emails and others didn't, as witnessme's comment seemed confused about this.
Do you reside outside of the EU (and outside anywhere where GDPR equivalents are enforced)? Maybe that would explain it.
Under GDPR, a business has the obligation to inform users if they have been affected by a data breach. That could hypothetically explain why Substack would inform some users (those protected by GDPRish legislation) while keeping it quiet towards the rest of them.