EDIT: I discovered how it is done. No, I will not share it here. It is an exploit for sure and anyone using it will most likely get a warning.
I was on a level 68 DH on one account and a level 49 DK on another account. Using the level 49 DK, I went to Chromie and chose the WotLK Timewalking Campaign. I then grouped with the level 68 DH and activated Party Sync. This effectively lowered the level and the gear of the DH to 60 (item level 174). I was then able to freely move around classic areas of WoW with Warmode enabled while slowly leveling up to 70, but still being restricted to 60.
Like a thunderbolt from the sky, a DH from the other faction landed on me and instantly killed me. I looked up and saw a skull where the level 60 should have been and 800,000 health. They waited around for me to resurrect and then instantly killed me again. While I sat on top of my corpse I decided to turn off party sync to make sure I wasn’t going insane. Sure enough, the player disappeared and the area became filled with level 30 mobs again. I reactivated party sync, and there they were.
Would someone mind explaining to me how a level 70 is able to bypass the Timewalking Campaign level restriction? I thought it was impossible to be ganked by level-capped players who have nothing better to do? And I’m positive they were not restricted because at level 60 they were appearing as a skull to me.