I would to like to visualize the time until the iPhone XR is no longer supported

Keep in mind while the 4s did receive an update, it hasn't been able to get iOS updates since iOS 9.3.5. That means it hasn't been able to be upgraded to iOS 10, 11, 12 and now 13. Your phone was released in 2018. Typically, Apple fully supports a phone for 5 years. You can reasonably expect your phone will be current in terms of iOS updates until likely 2023.

What to you mean by "supported"?

I would say the 4S since it was the oldest device to receive an update this year.