Asklemmy

The developer of Terraria promised to open source the canceled sequel if a petition could get enough signatures but then it did and he didn't release the code

As I understand it, something like that happened with the game, Alliance. It was a RTS where humans had to control all the units that the commander build and ordered around.

I learned about this, probably 15 years ago, and I never played, so a lot of this might be outdated or just plain wrong.

I think Microsoft published the game, and eventually abandoned it. It still had a thriving mod community but they were struggling due to the lack of source code and support. Apparently, years after working on the game, one of the designers happened across an Internet forum talking about the struggles they were having. The designer then went back to his bosses and said, "We abandoned this years ago, can I just give them the source code?" I guess they did and since then the mod started making HUGE fundamental changes to the game.

Freespace 2 comes to mind. The whole game engine was open source not just the game itself. Looks like there is still a community building on it too. https://scp.indiegames.us/

The history of city of heroes might interest you.

Not strictly the same, but one of the most amazing feats to me in this topic was done by the Sacred community over at DarkMatters.

Apoligies for the wall of text, but I consider it worth a read.

Sacred 2 in particular never had its server code open sourced, leaked, or anything of the like as the studio went bankrupt before anything could happen, this was around 2010.

Over the course of a decade a few volunteer devs would pick up a project where using tools like wireshark etc they'd essentially sniff traffic sent by a client attempting connection to a server that didn't exist, and using this, devs would literally try to GUESS what a server would respond, and what a client expected, essentially trying to build out the backend infrastructure from SCRATCH.

Fast forward to 2020 or so and progress was still being made, not only that but things were beginning to actually take shape. In 2021 (IIRC) one dev in particular had the general frame of a working server and continued to work on it. Fast-forward and since 2022-23, you're able to run both a LOBBY for multiple servers and an actual GAME SERVER yourself, self-hosted and code is open.

I've ran a couple servers using docker since, where I played with friends, and being able to replay that childhood game, with friends, one I thought I'd never be able to share the experience for, is a dream come true.

Another neat thing is that it was reverse-engineered in windows, but the docker containers literally run WINE to translate windows calls to Linux and it just works.

Knowing I'm able to in 2,5,10,30 years pick this up, and not only that, but replay with friends means this work of art has a great chance at preservation.

If you're into power metal, there's a band called Blind Guardian, they not only did they the main theme for the game, but the band's members have an entire quest-line in-game that culminates with an in-game concert. Again, a work of art worth preserving, and now, it can be shared.

While not open source, OldUnreal has taken over Unreal and Unreal Tournament with access to the source code and they release patches. The OldUnreal team has an agreement with Epic to do this.

https://github.com/OldUnreal/UnrealTournamentPatches

Hawken was recently brought back from the dead by a few insanely dedicated fans.

Does rewriting the game count? https://2009scape.org/

The server is a rewrite from scratch and the client is a decompiled and deobfuscated binary from January 2009

Does the Unity release of Daggerfall count?

Club Penguin Rewritten, Toontown Rewritten, and The Legend of Pirates Online (based on Pirates of the Caribbean Online) are resurrected fan-made versions of games shut down by Disney. The former was also shut down by Disney, but the others are still going. I think the difference is that the Club Penguin revival included ads while the other two were never monetized at all.

Magewars had a brief revival

Early Dooms and Quakes https://github.com/id-Software/DOOM but they aren't open source for the reasons you wanted.

Star Wars Galaxies.

Ur-Quan Masters (aka. Star Control 2)

But, it's not really abandoned anymore. The developers are FINALLY making an official sequel!

The sequel

The sequel is not open source, but UQM/SC2 is.

This happened in the early 2000s, but I think they found the source code to a port of the game and said "We haven't earned any money from sales of this game in a decade [and buying digital games wasn't really a thing yet, as people generally believed that anything digital shouldn't have a price], so let's release this to the community to open source as long as they do all the reverse porting and support!"

Warzone 2100 is an RTS that fans petitioned a company to release as FLOSS, after support had ended.

One that I'm aware of is Re-Volt. The source code for both the base game and some addons can be found here.

There was another one that I remember using years ago but I can't find it anymore. It was for another racing game called 4x4 Evo. There is the 4x4 Evolution Revival project but it's not the one I used and there doesn't seem to be any source code available.

!principia@sopuli.xyz was developed as a commercial title a few years back. I believe, @ROllerozxa@sopuli.xyz contacted the devs to get it open-sourced.

Not quite the same, but Total Annihilation and Beyond All Reason. It wasn't abandonware, but more like after Total Annihilation hit success, rights were sold and resold and Atari as the final owner squandered every opportunity to do more with the engine and the franchise.

The tactics were essentially receating a better engine with Spring, as the sort of newly open source upgraded version of the engine the same people built 10 years earlier. Taylor Swift did the same thing.

HoverRace!