Comment by araes

Or an alternative for the Sega Genesis https://github.com/Stephane-D/SGDK

Or the Super Nintendo Entertainment System https://github.com/alekmaul/pvsneslib

Or the Gameboy / GBC, Sega Master System, Gamegear, Nintendo Entertainment System https://github.com/gbdk-2020/gbdk-2020

Or the TurboGrafx-16 / PC Engine, Nintendo Entertainment System (alt), Commodore 64, Vic-20, Atari 2600, Atari 7800, Apple II/IIe, or Pet 2001 https://github.com/cc65/cc65

Or the ZX Spectrum, TRS-80, Apple II (alt), Gameboy (alt), Sega Master System (alt), and Game Gear (alt) https://github.com/z88dk/z88dk

Or the Fairchild Channel F https://channelf.se/veswiki/index.php?title=Main_Page

Note: Some are slightly pre-1999 (all these, I have at least successfully made a "Hello World" with)

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If they're really wanting 1999, that's the 5th to 6th generation console range with Sega Saturn, PlayStation, Nintendo 64, and Dreamcast. (on these, only recommendations, no successful compiled software)

Playstation is really challenging and remains so even in 2026. Lots of Modchip and disk swap issues on real hardware. Possibilities: https://www.psx.dev/getting-started and https://github.com/Lameguy64/PSn00bSDK

N64 is less horrible, and there's quite a few resources: https://github.com/DragonMinded/libdragon and https://github.com/command-tab/awesome-n64-development

Sega Saturn is still pretty difficult. However, there is: https://github.com/yaul-org/libyaul?tab=readme-ov-file and https://github.com/ReyeMe/SaturnRingLib plus the old development kits from the 90's are still around https://techdocs.exodusemulator.com/Console/SegaSaturn/Softw...

Dreamcast is similar to the Saturn situation, yet strangely, a little better. There's https://github.com/dreamsdk/dreamsdk/releases and https://github.com/KallistiOS/KallistiOS along with the official SDKs that are still around https://www.sega-dreamcast-info-games-preservation.com/en/re...