Hello. Does anyone know if there have been cases when proprietary software became freeware/open-source? I’m interested in authoring systems for older hardware: Spectrum, C64, etc.
For example, an article about Adventure Builder System lists its license as “Former Commercial”. Other similar systems have the license listed as “commercial”. Where can I find information about the legal status of such software? How important is it anyway? 
Tom’s ABS may be listed as “former commercial” but that doesn’t mean it’s freeware or open source. It’s just no longer being sold. Systems were generally sold with the ability to freely distribute (and usually sell) any adventures made with them, so as long as you own a legal copy of the system you are fine to produce games and sell them, even thirty-years on.
If you want to develop for Spectrum, C64 and the like then there are plenty of systems out there that will allow you to do so. The best, in order of complexity and features, are the Quill, the PAW (Spectrum & CP/M only) and DAAD. DAAD has been released for all to use freely and you can send Tim Gilberts some money by Paypal to do right by him if you want to use the PAW/Quill and don’t have a physical copy.
The other option is PunyInform, a variant of Inform 6, which is very powerful and great for C64 and a huge range of retro platforms. Not so good for Spectrum.
My systems of choice for 8-bit and 16-bit development are the PAW and DAAD. I usually develop/prototype on the PAW and then release across all the platforms by porting the game to DAAD. DAADready is a great tool that will allow you to develop and publish for multiple machines very quickly,
If you want to target the C64 specially, don’t forget Dialog !
What is it with Dialog, exactly? The last thing I heard, it was a domain-specific language based on prolog with a lack of any real games of note being created with it, aside from a few from the guy who created the system. No offense, I’m just out of the loop, as I’m normally a citizen of TADS land, though I do love the retro computing and IF scene almost as much as anyone else on this board.
Now it’s up to 17 games from about ten authors. (Says IFDB.)
It’s a low-traffic system but it’s not just Linus.
Zilf that compiles Infocoms old ZIL-format is also an option for retro systems that have an interpreter for the Z-machine (just like Punyinform).
That hasn’t been true for some time. I fell in love with it because it’s a joy to work with (and ZIL seems to have stalled). Now I just need to actually get the game I’m working on out…
Probably worth mentioning that, with how the computer gaming industry has evolved, the time spans involved, and that the platforms in question are kind of niche even within retro game circles(or at least, that’s my impression), many of the legal entities involved with retro development systems don’t exist anymore, much of the IP is probably lost in the digital ether, and any company that currently exists and has claim probably doesn’t care about people homebrewing on 40-year-old hardware. Doesn’t make it legal, but probably does make much of it fall into the gray area of “not legal, but who’s going to enforce?”.
Also, I’m sure googling [nameOfSystem] homebrew should lead you to resources for developing for old hardware, including stuff that’s still maintained, if there is anything. Recently stumbled on a youtube channel that talks quite a bit about lesser known machines from the pre-crash and post-crash 8-bit eras and the tricks both games of the time and homebrewers of the last decade have used to get around hardware limitations(in particular, I remember mentions of Bifrost, a bit of Spectrum code that could get around the 2-colors per 8*8 tile limit, though admittedly, that might be irrelevant for a text-only game).
The 6th place game in this year’s IFComp was written in Dialog.
(Forsaken Denizen is a Dialog game)
People sometimes use the term “abandonware” to describe software that’s proprietary and technically still owned by someone, but no longer commercially available, probably abandoned by its owner, and realistically unlikely to result in legal action if you distribute it.
The broader copyright concept is “orphan works,” which are still under copyright but whose owner can’t be contacted, might be unknown, or might not even exist anymore.
I think this is the real answer here. Legally, it’s still owned by some entity or another. Practically speaking, though, it’s unlikely anyone will care if you use or distribute it. And morally, it’s not like anyone’s selling it any more to buy legally, so you’re not hurting anyone’s income.
Sean Ellis, creator of the GAC and STAC shared his source code listing of GAC. Unfortunately, it is not clear whether the author/publisher gave permission to use the code and the program itself.