This happens to me several times a month. I'm more concerned about account termination, in that if their Gmail account is terminated for some reason, mine would be as well due to it being the backup email address.

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.

As far as I know, it only contains users who have made Substack profiles. Regular subscribers don't seem to be included, though I could be wrong.

I'd edit my other reply to this comment but can't anymore.

Here are the columns from the CSV file I've seen being shared around on forums, including the "internal metadata". This mostly boils down to full name on file, email, Stripe customer ID, activity metrics, usernames, and phone numbers. Everything else is largely irrelevant.

id,name,email,email_confirmed,email_confirmation_token,stripe_platform_customer_id,is_global_admin,is_ghost,created_at,anonymous_id,email_bounce_count,photo_url,publisher_agreement_accepted_at,bio,updated_at,profile_set_up_at,tos_accepted_at,email_digest_at,has_passed_captcha,import_confirmation_required,post_notification_preference,reader_installed_at,activity_items_viewed_at,dismissed_ios_app_promo_at,email_notifications_last_resumed_at,previous_name,release_group,handle,phone,bank_payment_failures,is_globally_banned,session_version

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.