When someone unsubscribes from a list, they are removed from that list. However, when someone unsubscribes from a flow or email sent to a segment, this person becomes suppressed in the sender's Klaviyo account. If you have Klaviyo's global unsubscribe feature enabled (which is highly recommended), this user is considered globally suppressed. Their profile will remain intact and appear in lists, but they will no longer receive emails.
For example, if your main Newsletter list has 500 people on it but 5% are globally suppressed, then when you send a campaign to this list, Klaviyo will automatically skip suppressed users and you'll notice 475 emails were sent.
Clean Global Suppressions from a List
If you wish to completely remove globally suppressed contacts from a given list, first, navigate to that list. Then, in the upper right-hand corner, click Manage List > Remove Global Suppressions.
A popup will appear asking you to confirm that you wish to remove global suppressions. Click Continue. Klaviyo will then remove these profiles from your list.