I will second everything that this slashdot comment says.
The WIN32 version, all that we've ever released, still work. Some of the older ones may not find old servers, but the code itself is functional.
None of the older linux versions work on a modern linux distribution. In fact, we gave up maintaining the Linux version because it was too much work to keep up with the drift in LSB, OpenGL, libc, and when you put in man-months to get the latest version working on the main distributions, you had the Gentoo users screaming bloody murder that we can't support every combination of compile flags they used. LSB was supposed to fix this, but it was a dumpster fire.
Mac is somewhere in between. Apps run for a while, but the deprecation of 32-bit apps broke older Google Earth binaries. On OS X, you need to recompile and re-distribute your software every 2-3 years, would be my rough guess.
Windows has backward compatibility solved better than anyone. I can still run 1990's code on Windows 10.