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.
replyIt'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.