I completely agree, let's hope we get some for XMas
In the meantime I just searched my Reg for "conquer" and got:
Command & Conquer™: Generals and Zero Hour x 3
Command & Conquer™ Remastered Collection x 5
C:\Program Files (x86)\Common Files\EAInstaller\Command and Conquer Generals Zero Hour\Cleanup.exe
G:\Origin\Command and Conquer Generals Zero Hour\generals.exe
G:\Origin\Command and Conquer Generals Zero Hour\
G:\Origin\Command and Conquer Generals Zero Hour\GDFBinary_en_US.dll
Command & Conquer™: Generals and Zero Hour x 2
https://www.commandandconquer.com
WINXPSP3 RUNASADMIN x 2
Command and Conquer Generals Zero Hour [key name]
G:\Origin\Command and Conquer Generals Zero Hour\Command and Conquer Generals\
Command & Conquer™ and The Covert Operations™
Command & Conquer Windows 95 Edition [key name]
FirewallRules [key name] x 8
C:\Program Files (x86)\Common Files\EAInstaller\Command and Conquer Generals Zero Hour\Cleanup.exe
While in there I also noticed one or more entries for:
Steam
GOG
Eidos
Electronic Arts
Epic Games
GameSpy [is that still around?]
Paradox Interactive
Petroglyph
Playrix Entertainment
Ubisoft
Valve
MS Solitaire
Firewatch
Hidden World of Art
Pingus
Conclusion:
The registry is a huge database so I'm not going to search anymore.
Obviously we can't say for sure, but my guess is almost all games do this.
Depends on how well the uninstall routine is written, which can/will vary by game. I can categorically say 'Not all are deleted'—which btw applies to all software.
Grr, you made me go back into The Labyrinth and search for "resistance", my recent Far Cry 5 mod:
An entry for the Desktop icon—I'm fairly sure this would have been made by file manager, not the mod.
G:\Steam\steamapps\common\FarCry5\FC5Resistance_v4.11\FC5ModInstaller.exe.ApplicationCompany x 2
G:\Steam\steamapps\common\FarCry5\FC5Resistance_v4.11\FC5ModInstaller.exe.FriendlyAppName x 2
G:\Steam\steamapps\common\FarCry5\FC5Resistance_v4.11\FC5ModInstaller.exe x 2
C:\Users\<UserName>\Desktop\FC5 Resistance Mod.zip
So clearly, yes some do. However, the Resistance mod is a lovely piece of work—a collection of many different mods, with a UI to enable individual ones, and also make option selections in some of them. So it's really a serious piece of software, and therefore more likely to adhere to Windows best practices.
I've used other single-app mods where it was a case of 'Replace the original 3 files with these 3', which had no Registry interaction.
My guess is that a mod which has an installer will write to the Registry—but copy/paste mods definitely [?] won't.
Does it matter?
Not an expert of course, but I say generally 'No.' Almost all software writes to the Reg, it's what Windows wants apps to do.
There was a time—90s & 00s—when Reg size was a bit of an issue due to weaker hardware, and it was worth keeping it tidy. But 10s hardware is fast enough that processing the dross leftover in the Reg from bad uninstalls costs nothing in practice.
Game Mods
In the specific case of game mods and other software hacks, who knows? Many mods affect the same files as other mods, and so won't work with each other—ie you can have one or the other, not both. If different mods write to the same place in the Reg, then they'll also conflict.
In an ideal world, a mod should tell you if it works with all, or what it conflicts with. But of course mod authors can't test against all other mods, so again who knows? If you're worried that installing a new mod might break current important ones, do system and game disk images beforehand, so you have a quick recovery route.