It’s stated that they won’t be gone forever like the ones from 2017. They will just be placeholders for updates that may come in the future, like remember the frogs?
THIS ONION WON'T MAKE ME CRY. MOJANG WILL NEVER BRING BACK ANY REJECTED MOBS, NO MATTER WHAT, EVEN IF THEY ARE DEARLY MISSED OR IF THEY WOULD ADD LOTS OF COOL FEATURES TO THE GAME..