Duckstation was licensed under GPLv3, which requires that not only downstream changes are shared upon request, but everything used to build that source too.

Arcade1UP refused to share changes they made to support some Simpsons arcade cabinet they were selling. They eventually shared the changes after being pressured, but even then the sources they shared weren’t in any decent or buildable state. Some additional context: https://old.reddit.com/r/Arcade1Up/comments/10osxo0/john_d_states_arcade1up_is_not_in_violation_of/

If I was maintaining a project the size of Duckstation I’d consider this my last straw too. He’s cleared the license change with all contributors so it’s his call to make.

Ultimately if the license is a sticking point the community can fork from the last known GPLv3 commit and continue work from there. Using Swanstation is also an option as that was forked a while ago.

Emulators come and go all the time, this is no different.