ConnectApi.PinCapability

If a feed element has this capability, users who have permission can pin it to a feed.

Property Name	Type	Description	Available Version
`isPinnableByMe`	Boolean	Specifies whether the context user can pin or unpin the entity to the feed (`true`) or not (`false`).	41.0
`isPinned`	Boolean	Specifies whether the entity is pinned (`true`) or not pinned (`false`) to the feed.	41.0

Apex Developer Guide