Everything I see from googling says that time travel is gone now on the Apple Watch. However, if I create a brand new watchOS project with complications, there's still a getTimelineEntries(for:after:limit:withHandler) method.
So if it exists, how do I use it now, since the setting seems to be gone from the watch app, and if it doesn't, why does the default template still include it?
While the Time Travel feature is indeed gone, Complications still use a timeline in order to have your complication's information available quickly, without launching the app. The remaining methods in the CLKComplicationDataSource protocol enable that.
This session from WWDC20 has some more information on timelines and why they're used. https://developer.apple.com/videos/play/wwdc2020/10046/