Blizzard added a new realm first achievement for Mythic+ in the latest Dragonflight Beta build. 

Players who participate in the realm-first Mythic +20 (or higher) clear within the time limit will receive it in Dragonflight Mythic+ Season 1.

Realm First! Dragonflight Keystone Hero – Participated in the realm-first completion of a Level 20 or higher Dragonflight Season 1 Mythic dungeon within the time limit.

It would be cool to see more realm-first achievements return in Dragonflight.