When will the iPhone 7 discontinue updates?

Why? major version updates for the 8 year old iPhone 5S and the 7 year old iPhone 6 were frozen at iOS 12.x in 2020, and that was only because the hardware could not support later versions. The 7 is only 5 years old. The controlling factor is whether the hardware is too old to support a newer version. So if you just go on past experience the 7 would age out in 2022.

No one in this user-to-user forum has any information that would answer your question. And anyone at Apple who knows isn’t going to tell you.