As of now they just started defaulting to a tip on the post-ride screen. But, you can still opt out.
Note that obscure methods of opting out may still result in a yes judgment of the default is to tip and it's not easy to opt out. By EOY 2024 broad release.
@Epsilon not easy, would count as force tipping
To disable forced tipping you have to email them to disable forced tipping
Or enabling developer mode or installing a dev app
Going through some kind of verification to enable not tipping
Going through multiple steps every time to stop the app making you tip
Even if the app allows you to manually reduce the tip to 0.01 or small amounts, that's still forced tipping
Easy, these changes would not count as forced tipping
Doing two clicks every time not one to skip the tip, either a hidden option or nag screen
Disabling a setting to prevent force tipping, one time, and the setting sticks