Some old software is put into public domain for the purposes of encouraging people to use it and then pay to upgrade.
IF your software is in this category, then I don’t suppose it makes any difference where you got it from, but you will need to check with the organisation that currently holds the rights to it to see what their position is.
If the software is not available in the public domain, then it doesn’t matter how old it is: it would be theft to use it, even if there’s currently no way of purchasing it. You would be expected to purchase the most up to date version instead.
As for fan-made films that ‘inhabit’ a fictional universe… again, that depends on the attitude of the rights holders. Typically they accept it (or tolerate it) provided the fans do not profit by it financially. This is the case with Paramount and the Star Trek franchise (I know this for certain, as I’m involved in just that sort of project in that ‘universe’). The clinchers are that it has to be entirely not for profit and, also, that the end result is identifiably made by enthusiasts and doesn’t purport to be professionally made: i.e. if it is SO good that it looks like Paramount themselves might have made it, then Paramount would likely object. But in general terms, fan-made stuff is made on such a shoe-string budget (and the acting is usually so dire!) that nobody in their right minds would confuse it for the real thing!