While Blizzard still hasn't provided a date for the pre-patch release, they did say that there will be two pre-patches instead of one.
A small part of the article explains that:
First, the expected pre-patch, 9.0.1 will arrive and bring features like the new character customization options and the streamlined leveling experience (and presumedly the level squish). Then, sometime very shortly before the actual release of Shadowlands, patch 9.0.2 will go live and bring the polished version of the level 51-60 leveling experience. This approach is designed to get the new character customization options and new leveling experience in the hands of players as soon as possible, while still allowing Blizzard to take a few more tweaks tweaking the level 51-60 experience.
The remainder of the article can be found here: https://gamerant.com/world-warcraft-shadowlands-two-pre-patches/