Write it in a specific version of a language with a strong history of backwards compatibility, like Fortran90. Maybe c99? And just use very famous libraries, like BLAS, that are never going away.

No idea what to do if it needs a gui, though.

Think you need to go with a "dead" language with a simple runtime and keep everything vendored. My top contender would be Lua 5.1. Simple runtime (~20k(?) lines of C) which has been implemented in many other platforms (Javascript, Go, Rust). The side benefit of 5.1 is while you can make the standard compiler your target, you can probably run on LuaJIT (not dead, sophisticated assembly, and potential for breaking changes) as well.

Try to run it on a 20-year-old system.

If it ran 20 years ago and it still runs now, it's very likely to still run in another 20 years.