I couldn’t find any noticeable difference in the Timer interface between iOS 26 (iPhone 12 Pro) and iOS 27 (iPhone 13 Pro). I have included a screenshot of the Timer interface on my device below for your reference.
You mentioned: “I have an iPhone 15 with iOS 27.0.1 installed yesterday. Is there any way to roll back the Timer interface? Specifically, the slider at the top.”
Based on my observation, I am unable to identify the change in the Timer interface you are referring to, particularly the slider at the top. Therefore, either I have misunderstood what you mean by the change, or your claim that the Timer interface has changed in iOS 27.0.1 may be incorrect.

Could you please post a screenshot of your Timer interface and point out the specific slider you are referring to? Comparing it with the screenshot I have provided should help us establish whether there is an actual difference between the two iOS versions or whether the issue lies elsewhere.