Essences will still work in the prepatch. If you wanna play bfa now you might as well get some essences. Especially if you already got them on your main.

What happens to essences in prepatch?

Rank 4 essences have been removed from the game and are no longer obtainable. These essence ranks are purely cosmetic, and will not affect your stats in any way. You can review the Shadowlands Pre-Expansion Patch notes to keep up with what has changed.

Are essences going away in Shadowlands?

While Essences won't be usable in Shadowlands, you will still be able to use it on any content taking place in Azeroth, so they will still be valuable while farming old raids while offering the extra cosmetic effects. ... Well, make sure to check our guide on What to do Before Shadowlands.