Linked key expiration triggers.

#15

Linked expiration works for:
Triggers
Absolute expiration

But not for:
Sliding expiration
Manual removal
Memory pressure removal
I'd honestly use a simple [ThreadStatic] rather than something in the call context.  The call context has potentially unwanted behaviors like being captured by async methods.  This could cause implicit links to be present in places where you didn't anticipate them.  It could also cause objects which you intended to be short-lived to survive GCs.