Board Meeting Notice via WhatsApp

Yes, it can be send.

Do you have any evidence of the same ?

Proof Of Delivery To Be Maintained ,If Whatsapp Has such Mechanism then You can send ! But I think once chat delete there is no proof of delivery can be maintained ,so if you can snapshot the blue tick of delivery in printable format ,may be it's possible ,but still email is considered safer and reminders can be send through WhatsApp !