Short answer
HubSpot's KB: after a suspension, blocklisting or trust and safety issue, HubSpot may require you to opt out or quarantine contacts. If a quarantined contact replies to a one-to-one or sequence email, or submits a form, the quarantine is removed.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
The KB explains which property to edit if you still need these contacts in sequences or one-to-one email.