I told my boss I had an idea for a program that could improve efficiency across much of the business, and he let me build it on company time. In the long term, he wanted to be able to sell it to other companies. However, the program never got implemented due to personnel mismanagement, and I’d rather be able to post it on my github under a free licence so I can use it as a resume item, and at least someone would have the chance to actually use it. It’s all still in my head, and I could write it again if I wanted. If I do, is it illegal to publish it? What if I write it in a different language? Do I need to change the variable names? I did plenty of research and planning on company time to build it, and it’s not like I can research it again, it’s all still in my head.
If you just care about the resume, then write on your resume: “have experience in doing this sort of stuff.” Recruiters aren’t going to look into GitHub to study your code.
If you care about getting the code used or sold, I’d first consider finding the next potential client to make use of whatever the program does. Their need will likely be different, so you’ll need to rewrite it anyway. You then have both the client and a new product on your hands.