Retro a lot of R4 flash carts used by licensed DS developer “The Code Monkeys” have surfaced online (includes 3 unreleased titles)

[Krvavi Abadas]

(the three unreleased games. Tiny The Mole, Maniac Monkey Mayhem, and King of Clubs.)​

yes, those R4 cards. the ones Nintendo took legal action on back in the day over piracy concerns.
of course, the usage of devices like this as gray market development kits is nothing new. Acclaim, one of the biggest 3rd party developers on the N64. had numerous Doctor V64 copiers stashed in their office for this purpose.

it was a much easier solution than creating custom hardware from scratch, which occasionally happened (even by Nintendo themselves) during the SNES era.

however it should be noted that Code Monkeys evidently wasn’t solely using the R4 for development, as alongside the prototypes and development tools (both in-house and stock ones taken from the official DS SDK) there’s a pretty sizable collection of pirated NDS roms included. so do excise a bit of caution when linking the archive.
some of the builds (most notably Tiny the Mole, which “burrows” music from Clubhouse Games of all things) also use assets from unrelated commercial DS games, but that sort of thing is fairly common as a placeholder.

 

[Hero of Legend]

Believe it or not... The Code Monkeys were behind those infamous Phoenix Games games (just the games, the infamous animated bits were by Dingo Pictures IIRC).

 

[Dcubed]

Almost every single western developer who worked on GBA or DS games used one of those illegal flash carts/cards (Yes, even Wayforward). They were outright ubiqiuous amongst the development community because they were far cheaper and easier to get a hold of than Nintendo's official development flash cartridges, and usually more feature-rich.

If they're functionally identical? Why not just pick the easier and cheaper option? It makes no difference to the end product.

On a slight tangent, but still related, one of the biggest reasons why the PS1 won its own generation was because Sony bought out Psygnosis; who owned SN Systems, the creators of the SNASM SDK. SNASM was THE SKD of choice for just about every single 3rd party developer back in the 16bit era, and more or less every single western developer chose it over the official SDKs from Nintendo & SEGA. Sony bought them out in 1993 and got them immediately working on the PS1 SDK Psy-Q; making it the default SDK for all PS1 developers. Having an immediately intuiative and affordable set of SDK software and hardware before your console even launched was amongst the biggest reasons why developers flocked over to the PS1 over its competition.

 

[Razgriz]

NERD (Nintendo of Europe's tech subsidiary and the main developer for non-game stuff at Nintendo) even used DS flashcards for internal testing if memory serves me right.

We found debug images of one of the DS flashcards in one of the leaks from a few years ago. IIRC it was the one about Wii U's DS emulator.