It looks like Timewalking in World of Warcraft Shadowlands will require you to be level 50.
This is after the changes to leveling in general. As Wowhead notes, premade group finder reflected this level requirement as well. Due to the level squish, level 50 will be the new Battle for Azeroth max level moving forward. This means that any level 120 players will be squished to level 50.
Previously, if you had leveled past the level for an expansion, timewalking was a way for those players to access that content. It sounds like when you ding level 50, you’ll be able to access premade group finder and timewalking. For example, Timewalking dungeon events would be available for short periods of time allowing you to assemble raid groups, and more.
You can catch a glimpse of how this looks in the beta on Reddit. Additionally, in case you missed it, Shadowlands was delayed out of October. It’s set to release sometime this year, but no date has been provided yet.
Finally, a recent development update outlined changes you’ll expect in the beta including the Conduit system arriving in the next build of Shadowlands.