up for grabs.

Got the covid-19 vaccine

Something equally accesible by any legitimate participant.

Freely available, usually used in a sexual sense. Comes from English Grouse hunters expression to identify a bird that is a legitimate target as opposed to a bird that is injured, too young etc.

available, desirable, ok to approach, or to speak of

Girls who are up for grabs.

An opportunity, that might be presented by another's carelessness.
An opportunity which can be taken without repurcussions.