Shadowban
Definition
Reach restriction applied by a platform without notice to the affected account, which reduces distribution of the content without removing it or blocking access.
A shadowban is a sanction with no notification. Reach drops, the account stays open, and support has nothing to say about it.
Compare several weeks of data before assuming one. External links, flagged words and recycled content are the usual causes, roughly in that order.
Keep your own metrics. When a platform does not acknowledge a restriction, they are the only evidence in existence.