How would I make a decoder agnostic steganographic file? I would like to make a steganographic file that can be decoded on multi-platforms, now &amp; 20 years from now.

I don't want to do so with software that may be out of date when the message decode is attempted.

How would I do this?
I will assume you're referring to steganography in images.

The most important thing would be to use a color space that is likely to be recognizable 20 years in the future. For this, sRGB is a good candidate. It was created more than 20 years ago, in 1996, and it is still the most popular color space in use. It's safe to bet that, in 20 years, it will not be forgotten. You can use traditional LSB steganography to encode information. As long as the color space is preserved, it should be compatible with whatever future software you are using (or compatible software that is easy to find).