This is partially on third-party devs' shoulders. There are community-maintained backports of Chromium and Mozilla for old versions of macOS, which means it isn't strictly impossible, but Google and Mozilla just don't bother and cut off compatibility earlier there than on other platforms.
It's even possible to build Mac apps to run on everything from OS X 10.4 (2005) through macOS 27 (2026) across PowerPC, Intel, and Apple Silicon in a single binary. See XLD[0], which does exactly that.
Command-line tools are fine like this; a full-fledged GUI apps, not so much on account of how Cocoa/AppKit/UIKit evolved. Your x86_64 slice would have to deal with all shenanigans between 10.7 and 26, and I suspect it’s not fun at all.
It's certainly more challenging for user-facing apps with UIs, but still doable. XLD does it. The key is to keep the UI simple, avoid chasing design trends, and avoid custom widgets.
It's even possible to build Mac apps to run on everything from OS X 10.4 (2005) through macOS 27 (2026) across PowerPC, Intel, and Apple Silicon in a single binary. See XLD[0], which does exactly that.
[0]: https://tmkk.undo.jp/xld/index_e.html