1.
You can say that an event or situation is unheard of when it never happens.
Meals are taken communally with other guests in the dining-room. Private bathrooms and toilets are unheard of...
It’s almost unheard of in France for a top politician not to come from the social elite.
ADJ : v-link ADJ
2.
You can say that an event or situation is unheard of when it happens for the first time and is very surprising or shocking.
Mom announced that she was going to visit her family for a couple of weeks, which was absolutely unheard of.
ADJ