The new block-based navigation screen is once again delayed after it was originally slated for WordPress 5.5 and then put on deck for 5.6. Contributors have confirmed that it will not be landing in WordPress core until 2021 at the earliest.
“The Navigation screen is still in experimental state in the Gutenberg plugin, so it hasn’t had any significant real-world use and testing yet,” Editor Tech Lead Isabel Brison said. She made the call to remove it from the 5.6 lineup after the feature missed the deadline for bringing it out of the experimental state. It still requires a substantial amount of development