PCSX2 (a PlayStation 2 emulator for the PC)
Windows, Linux and MacOS from what I can tell
Some changes listed:
GUI
You might know the name wxWidgets from its time as PCSX2's choice of GUI, or from other emulators like Dolphin which used it before migrating to Qt. For a long time, it served us sufficiently well, but its age and implementation began to show rather quickly. Threading issues resulted in deadlocks and race conditions, because it was not properly separated from the emulation core. The technical debt started high and only got worse as the project tried to keep moving forward.
But now, wxWidgets is gone. No more desktop experience reminding you of what Windows desktop apps looked like in 2006. Qt brings with it a slick appearance, more efficient UI backend, and provided the perfect opportunity to redo all our menus and widgets.
Settings / Config
But then we had a thought: since we are already shipping a database file with information on games, why not include more information about what settings a game needs to run correctly? And so PCSX2's new "game index" was made a reality. It contains a complete list of all known games, and with this index, we now ship pre-configured fixes to ensure your games will automatically run smoothly. You no longer need to remember to enable those graphics fixes every time you switch games - PCSX2 will already know what that game needs and do it for you!
Custom Settings / Config
PCSX2 now includes per-game settings which are detached from your global settings. The per-game settings will always default to inherit from your global settings, but they allow you to explicitly set a value for one game in particular. Now if you want to run a game at a higher resolution than the rest or use specific memory cards for different games, you can set it once in your per-game settings and forget about it.
Compatibility:
Right now, the list of games which aren't playable has been whittled down to a small handful with servers which have gone offline, highly obscure peripherals that no one has replicated in emulation yet, bizarre FPU math causing games to break themselves, or over-engineered engines which would bring even the best PC hardware to a literal crawl if it were emulated correctly.
All that remains of the 'Nothing' category is a single game which refuses to boot on Windows. Boot a game in PCSX2 besides that one, and it will at least get you into a menu. Provided no one uncovers any more obscure PS2 games that have been lost to time, it is safe to say PCSX2 has entered the final frontier of compatibility. The challenge now is to get the last few games up and running.
And others I haven't read through yet