Since 2024 is the 5th year anniversary of the 7,1 machine (2019) I would assume this year would be the last, but I could be wrong.
I am pretty sure Apple can't wait to dump support for all Intel machines and clean up the code and get rid of Universal apps and dump Rosetta 2.
I almost get the impression you can't wait for Apple to dump Intel support...
I think you overestimate how much Intel-specific code is in macOS and the impact it has on Apple Silicon performance. At this point macOS is a 20-years mature, multi architecture OS having run production on three architectures (four if you count NeXT). Processor-specific code should be pretty well localized. It really should be just limited to parts of the kernel and devices drivers and key libraries/frameworks (e.g. Accelerate). And it's all already written and shouldn't need to be changed. Nor does it appear that Apple is holding Apple Silicon back for the sake of feature parity between the two platforms.
The other heavily processor-specific component is the compiler. Since they use LLVM and x86_64 probably has the greatest market share of LLVM users, I doubt that team has any interest in dropping support for it.
I can see why they would want to drop Rosetta as its already served its main purpose and its value declines ever day. However doing so before they drop the whole Intel platform would really screw with smaller developers' ability to support legacy customers. Plus I believe Parallels/etc and Docker use it to support Intel binaries from those platforms and dropping support for it just excludes whole classes of users from the Mac platform.
Again already written and working. About the only thing missing is AVX/AVX2 support but at this point there's no real upside to adding that so my guess is they will just leave alone until the end. Maybe move from a general user optional download to a developer optional download (e.g. into some version of Xcode tools).
Then every version of macOS since at least 10.4 has had some sort of Universal app support and doubt they will actually remove Universal support from the platform. They will just drop the ability to generate new Intel binaries in some future version of Xcode meaning it will only be able to generate Universal binaries for one architecture. But I doubt that will be noticeable to Apple Silicon users (except perhaps somewhat smaller application sizes).
With Venutra, Apple dropped support for all systems with pre-Skylake processors along with I am sure lots of other legacy hardware. Of the remaining platforms (I count 10), the Mac Pro is probably the most complicated to support but the one that should be the last to be dropped based on both its last retail sale and nature of its customers.
If I were a betting man, I'd say these platforms will drop out of macOS before the Mac Pro 2019 (in order of likelihood):
-iMac Pro
-MacBook Air8,x
-MacBook Pro15,x